Polychora
"As for the analogous figures in four or more dimensions, we can never fully comprehend them by
direct observation. In attempting to do so, however, we seem to peep though a chink in the wall of
our physical limitations, into a new world of dazzling beauty."
H. S. M. Coxeter, in the
preface of "Regular Polytopes".
The generalization of "polygon" in 2 dimensions and "polyhedron" in 3 dimensions to any number of
dimensions is a polytope. A
4dimensional polytope is a polychoron (plural "polychora"). This is a finite
region of 4D Euclidean space* bound by at least 5 polyhedral 3faces, or cells; it must also have
at least 5 vertices, 10 edges and 10 faces. Edges link two vertices; likewise, cells meet in pairs
along their common faces. Faces and cells meet in equal numbers and in sets of at least 3 each at
each edge in a way specified by its polygonal figure (henceforth "edge
figure"). Just as each polyhedral cell is limited by at least 4 faces, 6 edges and 4 vertices, at
least 4 cells, 6 faces and 4 edges meet at each vertex in a way specified by a single polyhedral figure (henceforth "vertex
polyhedron"). These surface elements are, together with the inner 4D ``volume", part of the
polychoron.
Regarding the vertex polyhedron: Its faces correspond to vertex figures of the cells of the
polychoron that touch that vertex. Its edges correspond to faces of the polychoron that touch that
vertex. Its vertices and vertex figues correspond to the edges and edge figures of the polychoron
that meet at that vertex.
If two polychora A and B are duals, then to each vertex of A corresponds a cell in B, which is the
dual of the vertex polyhedron of A, and viceversa. This implies that the cell's vertex figure
(i.e., the face of the vertex polyhedron) is the same for two dual polychora! Each edge of A goes
through a perpendicular face of B, which is identical to that edge's figure, and viceversa. Two
dual polychora must necessarily have the same symmetry.
* This is, again, a traditional definition that excludes honeycombs, which are infinite
divisions of a 3D Euclidean space. However, polychora are closely associated with tesselations of
the 3sphere, which are also
finite.
Projections
The models below are orthographic projections of
polychora to 3dimensional space that can be built with the Zometool system, i.e., that are
"Zomable" (if a polychoron has a Zomable projection, we will likewise say it is Zomable). The
projections shown below are done along a vector that is perpendicular to our own 3D ``plane''.
In these projections, most or all elements are necessarily distorted, in the same way
that the projection of a Cube to a flat surface will necessarily distort most or all of the faces
and edges of the Cube. A way of seeing which elements are not distorted is by their colour:
green struts in the models containing them, blue in the models without (i.e., the colours of the
longer struts in the model). This only applies, of course, to cases where all edges of the
polychoron being projected have the same length, which is true for all polytopes in this site.
Such undistorted elements thus lie in 3D ``planes'' parallel to our own. If undistorted cells
occur, they invariably appear at the center of the projection; they are the ``closest'' and
``farthest" away from our 3D ``plane'', and the projection is ``cellfirst''. Below, we also show
``vertexfirst'' projections (with a connector at the center), which show undistorted vertex
polyhedra. Occasionally we show other types of projections.
If larger undistorted polyhedra occur in the projection, they represent cases where specific
sections of the polychoron (by additional 3D planes that are parallel to ours) happen to be
highlighted by its edges. An equivalent of this in 3D would be the flat Pentagons we can see
surrounding each vertex in the Icosahedron. If one of these envelops the whole projection, then it
is an ``equatorial'' polyhedron, the section by a 3D ``plane'' that is halfway between two opposed
central elements. An equivalent of this in 3D are the equatorial Polygons of the rectified
polyhedra.
All regular polychora have Zomable projections, below we show all of them. In addition, we show a
few Zomable projections of some closely related forms. Since all these projections have a lot of
important internal structure, there is no alternative to building them in Zometool: a paper model
would hide the internal structure of the projection. We will refer to the specific projections by
their 3D symmetry: thus, a projection with Tetrahedral symmetry will be described as a Tetrahedral
projection; and likewise for all others.
Regular and uniform convex polychora
In a regular polychoron all cells and vertex polyhedra are regular. This implies that it is
isochoral: the cells are not only identical, but the polychoron looks the same seen from all
of them; this implies, for instance, that all cells are equidistant from its centre. The same is
true for the other types of elements, i.e., the polychoron is also isogonal, isotoxal and isohedral.
There are sixteen regular polychora: six are convex and ten are nonconvex. Some have many
names  one is known as the 120cell, Dodecaplex, Hyperdodecahedron, Polydodecahedron,
Hecatonicosachoron, Dodecacontachoron and Hecatonicosahedroid. The convention adopted here is the
same adopted by Wikipedia, which is to use the simpler names, for this example "120cell". This is
consistent with the names of the regular polyhedra, which refer only to the number of faces, not
their type. This simple system works because the resulting names are unambiguous. Furthermore,
"120cell" is much easier to understand, and shorter to write, than the equivalent Greek name,
"Hecatonicosachoron". The only exception is that for the 8cell we will use the widely known term
"Tesseract". This is also consistent with the polyhedral and polygonal names, where the terms "Cube"
and "Square" are at the same time technically precise (they imply regularity) and do not reflect the
number of faces or edges.
Apart from the regular polychora, we will also show a few of their rectifications (but not of their
duals). These are uniform
polychora: they are isogonal and all cells are uniform polyhedra, which are also
isogonal. But apart from that, these rectifications are also isotoxal, which implies that all vertex
polyhedra, apart from being identical, are also isogonal. The number of their vertices is the same
as the number of edges of the regular polytope that was rectified. One of the differences is that,
while for each pair of dual regular polyhedra there is one rectification, for two dual regular
polychora (or any higher dimensional polytope) there are two distinct rectifications.
***
The 5cell is the 4D analogue of the Tetrahedron. It is bound by 5 Tetrahedral cells and has 5
vertices, with Tetrahedral vertex polyhedra. These cells have 5 × 4 = 20 Triangular faces,
however, each face is shared by two cells, so the total numbers of faces is 10. The cells also have
5 × 6 = 30 edges, however, the edges have Triangular figures, meaning that each edge is shared
by three cells, resulting in a total of 10 edges. The symmetry of the numbers of elements
(510105) and the duality of the Tetrahedral cells and Tetrahedral vertex polyhedra imply that
the 5cell is, like the Tetrahedron, selfdual. Its Tetrahedral projection is shown in Fig. 15.
Fig. 15: Projections of the 5cell and its rectification.
Left: A "vertexfirst" and "cellfirst" Tetrahedral projection of simplest regular polychoron, the
5cell.
Right: A Tetrahedral (thus, cellcentered) projection of the Rectified 5cell.
For each of the 5 Tetrahedra of the 5cell, the Rectified 5cell has 5 rectifications of the
Tetrahedron, or Octahedra. Each vertex of the 5cell is replaced by its vertex polyhedron, a
Tetrahedron, two of each touch the new vertices from opposite directions. The number of these new
vertices is the same as the number of edges of the 5cell, 10; their vertex figure is the Triangular prism because of
the Triangular vertex figures of the new Tetrahedra, the Square ``sides'' are the vertex figures of
the new Octahedra. Since all its cells are regular, the Rectified 5cell is one of the three Semiregular convex
polychora. Together with the Triangular prism, it is a member of the finite
multidimensional k_{21} family of Semiregular
polytopes.
Both polytopes in Fig. 15 have Pentachoric
symmetry. For more Zomable projections of the 5cell, see Fig. 35.
***
The 16cell, the 4D analogue of the
Octahedron, is bound by 16 Tetrahedral cells and has 8 vertices with Octahedral vertex polyhedra.
Following the logic used above to calculate the number of elements of the 5cell, we find it has 24
edges with Square figures, and 32 Triangular faces. Its dual, the Tesseract, is the 4D analogue of the Cube. This
is bound by 8 Cubic cells and has 16 vertices with Tetrahedra as vertex polyhedra. It has 32 edges
with Triangular vertex figures and 24 Square faces.
In Figs. 16a and b, we show in the front rows different Octahedral projections of the 16cell and
Tesseract. In the back rows we show projections of their rectifications, specifically the
rectifications of these specific projections. For more Zomable projections of the 16cell, see Figs.
28a and b. For more projections of the Tesseract, see Fig. 30.
Fig. 16a: Models of vertexfirst projections of the 16cell, Tesseract and their rectifications.
Front Left: The 16cell. Note the
envelope, which represents the equatorial polyhedron, the Octahedron. Three flattened projections of
the Octahedron also appear in the three perpendicular planes passing through the centre.
Front Right: The Tesseract.
Back Left: Rectification of the previous projection of the 16cell. More about this object below.
Back Right: The Rectified
tesseract. Each of the 8 Cubes of the Tesseract is replaced with rectifications of the Cube,
Cuboctahedra. Each of the 16 vertices of the Tesseract is replaced by the Tesseract's vertex
polyhedron, the Tetrahedron. Two of these Tetrahedra appear in the centre of the projection.
Fig. 16b: Cellfirst projections of the 16cell, Tesseract and their rectifications.
Front, left: The 16cell. This projection is especially good for highlighting its 8 vertices
separately.
Front right: The Tesseract. In this projection, two Cubes, one "near" and the other "far" from our
own 3D ``Plane", appear perfectly superposed and without distortion. The other six Cubes are
flattened into the six Square faces. This can be understood very well by imagining the shadow of a
Cube in a 2D surface, where it can be made to resemble a Square, bordered by flattened faces.
Back right: Rectification of the cellcentered projection of the Tesseract. Notice the six flat
projections of Cuboctahedra at the surface of the model.
Back left: Rectification of the cellfirst projection of the 16cell.
Note: the two projections of the Rectified tesseract in this and the previous Figure have been
disassembled.
We described above the meaning of duality between polychora. We now describe the meaning of duality
between projections. The cellfirst projection of the Tesseract in Fig. 16b is the dual of the
vertexfirst projection of the 16cell in Fig. 16a; the reason is that each cell of the former is
centered exactly on a vertex of the latter, and viceversa. Equally, the cellfirst projection of
the 16cell in Fig. 16b is the dual of the vertexfirst projection of the Tesseract in Fig. 16a. Two
dual projections necessarily have the same symmetry.
These regular polychora have analogues not only in 3D, but also in any dimensional space. The
analogues of the 5cell are known as Simplexes, all of them selfdual. The Tesseract is
the 4D Hypercube, and the
16cell is a 4D Orthoplex.
These polytope families are infinite. Among these, the 16cell is especially interesting: its
vertices represent half of the vertices of the Tesseract, making it, together with the Tetrahedron,
a member of yet another infinite family of polytopes, the Demihypercubes. Also, as one might expect,
Tesseracts can fill 4D
space, but the 16cell
can do that as well: it is the only member of this infinite family that can fill the Euclidean
space where it exists! The vertex arrangement of this honeycomb is known as the F_{4} lattice. If a 3sphere is centered on the vertices
of this lattice, the resulting arrangement is the densest possible regular sphere packing in 4 dimensions, with kissing number 24.
The four polytopes in Figs. 16a and b share the Hexadecachoric
symmetry, but one of them (the Rectified 16cell) has a higher degree of symmetry. That this
object is special can be seen in a detail in Figs. 16a and b: while the two projections of the
Rectified tesseract are different (the first is centered on a Tetrahedron, the second is centered
on a Cuboctahedron) the projections of the Rectified 16cell in both Figures are identical. We now
discuss this object in more detail.
***
Ultimately, the Rectified 5cell owes its semiregular status to a fact we mentioned when discussing
polyhedra: the rectification of a Tetrahedron is another regular solid, the Octahedron. This means
that, when we rectify any regular polychoron with Tetrahedral cells, we obtain a semiregular
polychoron bound by Octahedral cells and cells identical to the former's (regular) vertex
polyhedron. As seen above for the Rectified 5cell, the vertex polyhedra are equilateral prisms,
with a "base" given by the edge figure of the regular polychoron that was rectified.
Things are even more interesting when we rectify the 16cell. Doing this, we create a polychoron
where 16 Octahedral cells result from rectifying the Tetrahedra of the 16cell (the projection in
Fig. 16a is centered on these) and where 8 new Octahedral cells are the vertex polyhedra of the
16cell (the projection in Fig. 16b is centered on these). The vertex polyhedron is the Square
prism, i.e., the Cube. This means that this polytope is regular: the 24cell (see Fig. 17). This is the reason why the
projections in Figs. 16a and b are identical. There is an even deeper, and rather beautiful reason
for this, triality, which is a result of its Icositetrachoric
symmetry.
Fig. 17: Left: Yet again, an Octahedral projection of the Rectified 16cell, more commonly known as
the 24cell. Note the equatorial
polyhedron, the Cuboctahedron. Note too the three other flat projections of Cuboctahedra in three
orthogonal planes passing through the centre.
Right: An Octahedral cuboctahedralcellfirst projection of the Rectified 24cell.
How to build: Have a look at the Eusebeia page on the Rectified 24cell. For this
projection, see the part on the "Cantellated 16cell", for the pyritohedric projection, see the
first part of the page. This model has been disassembled.
Because it is a rectification of the 16cell, the number of its vertices has to be the same as the
number of edges of the 16cell, also 24. The number of edges (which have Triangular figures) and
Triangular faces is 96. All of this implies that the 24cell is selfdual. In all Euclidean
ndimensional spaces, this is the only regular convex polytope with this characteristic that is not
a Simplex. Furthermore, the 24cell and its symmetry have no analogs in any other ndimensional
space, i.e., the 24cell and its symmetry are clearly exceptional objects! We will now find
why.
The previous way of building the 24cell (by rectifying the 16cell) is known as Cesàro's construction; this is the
4D equivalent of obtaining a Cuboctahedron from an Octahedron by rectification. The "dual"
construction of the 24cell (Gosset's construction) is equivalent to the
construction of a Rhombic dodecahedron from two Cubes mentioned above. This construction illustrates
especially well why the 24cell is unique to 4 dimensions.
The first step is the division of a Tesseract into 8 Cubic pyramids. Their 4D volume is limited
by a Cubic "base" (one of the previous cells of the Tesseract) and by 6 Square pyramids, the
"sides", the apex lies 1/2 edge length above the base. In a second step, we separate these Cubic
pyramids and "glue" their Cubic bases to the Cubic cells of a second Tesseract, making those  and
all Square faces  disappear. Square pyramids from two neighbouring Cubic pyramids now contact each
other through what were their Square bases, thus forming 24 Octahedra  the cells of the 24cell.
The apices of these pyramids are 1 edge length from the center of the former Tesseract.
As shown by the construction of the Rhombic dodecahedron from Cubes, similar constructions can be
made in any dimensional space. The difference is that among hypercubes in all ndimensional spaces,
the Tesseract is the only one where the distance from the vertex to the centre (given by √ n /
2 edge lengths) is 1 edge length, i.e., it is radially equilateral. This means that the Cubic
pyramids and their Square pyramid "sides" used in the previous steps are equilateral, a necessary
precondition for building Octahedra.
To the 16 vertices of the original Tesseract, all 1 edge length from the center, 8 new ones were
added: the apices of the Cubic pyramids, which lie "above" each Cubic cell of the original Tesseract
also 1 edge length from the center. Thus, the 24cell is, like the Tesseract, radially equilateral.
These 8 apices have the vertex arrangement of the dual of the Tesseract, the 16cell. Thus, the
24cell has the vertices of a dual compound, the Tesseract and the 16cell. Furthermore, as we've
seen above, we can divide the vertices of the previous Tesseract into those of another two 16cells,
therefore the vertices of the 24cell can be decomposed into three sets of vertices of the 16cell.
As we will see, a set of 3 16cells with the vertices of the 24cell is a regular polychoron compound (see Figs. 29a, b and c).
Note that, although all Square faces disappeared in the last stage of Gosset's construction, their
vertices and edges are still present in the 24cell as the Square ``equators" of the Octahedra.
Since each Octahedron has three such equatorial Squares, the edges and vertices of the 24cell are
the edges and vertices of three different Tesseracts, which together make another regular polychoron
compound, the dual of the aforementioned Compound of three 16cells (see Fig. 30).
As suggested by their derivations, the Cuboctahedron and Rhombic dodecahedron are the closest
polyhedral analogues to the 24cell. The Cuboctahedron is the envelope of the cellfirst projection
of the 24cell in Fig. 17 because it is the application of Cesàro's construction
(rectification) to the Octahedral envelope of the projection of the 16cell in Fig. 16a. Being
represented by the green struts means that it is undistorted and that its 12 vertices (half of the
vertices of the 24cell), edges and faces are in the same 3D ``plane'' parallel to ours; being an
envelope of the projection means that it is the ``Equatorial'' polyhedron of the 24cell, which
means that its 3D plane contains the centre of the 24cell. The lines going from this centre to the
vertices of this Cuboctahedron thus appear, like its edges, undistorted; this implies that the
Cuboctahedron must be, like the 24cell, radially equilateral (see Fig. 27). The Cuboctahedron also
has an equatorial polygon, the Hexagon, also with 1/2 of its vertices, which is also radially
equilateral. Apart from the Tesseract and 24cell, the Hexagon is the only regular polytope with
that property.
The Rhombic dodecahedron is the envelope of the vertexfirst projection of the 24cell (see Fig.
29b) because it is the application of the Gosset construction to the Cubic envelope of the Tesseract
in Fig. 16b. Like the Rhombic dodecahedron, the 24cell can fill space. To see this, we apply the
same process used above to create the
Rhombic
dodecahedron honeycomb from the Cubic honeycomb to a Tesseractic honeycomb: First, we
divide half the Tesseracts of the honeycomb (which touch each other only at their vertices) into 8
Cubic pyramids. Second, we attach their Cubic bases to the Cubic cells of its 8 neighbouring
Tesseracts, thus making 24cells via Gosset's construction above. This is the regular Icositetrachoric honeycomb.
Its dual must also be regular; it is in fact the 16cell honeycomb mentioned above. Thus, it
becomes clear why 4D space is the only Euclidean space in any dimensions with three regular
honeycombs; all others have only their respective Hypercubic honeycomb!
***
As we've seen, the 24cell is unique to 4 dimensions. As we will see now, its existence is a
necessary condition for the existence of the more complex regular polychora, being thus fundamental
for understanding the exceptional richness of the fourth dimension. The construction of the more
complex polytopes from the 24cell is known as Gosset's construction, which we now describe (see
Coxeter 1973).
Step 1: Rectify the 24cell, obtaining the Rectified 24cell (Fig. 17). For each of the 24
Octahedra of the 24cell the latter has one Cuboctahedron (the rectification of the Octahedron). For
each of the 24 vertices of the 24cell, the Rectified 24cell has a Cube (the vertex polyhedron of
the 24cell). This polychoron has 96 vertices, one for each edge of the 24cell.
Step 2: Diagonally bisect all of its Squares. If the diagonals are chosen correctly, then
each of the 12 vertices of the Cuboctahedral cells has 5 edges and five triangular faces converging
on it, i.e., it has been transformed into an irregular icosahedron (see Fig. 18). The Cubic cells of
the Rectified 24cell have been transformed into sets of 5 Tetrahedra, with only the central one
being regular.
Fig. 18: Being a rectification of the Octahedron or Cube, the Cuboctahedron (in green, see also Fig.
4) has 12 vertices, the number of edges of the Cube and Octahedron. This is the same number of
vertices as the Icosahedron. That is why their duals are both dodecahedra, respectively the Rhombic
dodecahedron (Fig. 4) and the Dodecahedron (Fig. 2).
The reason for the identical number of vertices is that if we add new edges along the diagonals of
the Square faces of the Cuboctahedron as shown here (in blue), it becomes an irregular Icosahedron,
with 5 edges and 5 triangles converging on each of its vertices. By shortening those diagonals to
the size of the other edges, we transform the irregular icosahedron into a regular one. None of
these operations changed the number of vertices.
Step 3: Shorten these Square diagonals until they have the same length as the other edges.
This generates one regular Icosahedron for each of the previous 24 irregular icosahedra, and the 24
clusters of mostly irregular tetrahedra become clusters of 5 (regular) Tetrahedra, 120 in total. The
resultant polychoron is the Snub 24cell (Fig. 19). The second and third operations preserved the
number of vertices of the Rectified 24cell, 96, all of them are still identical, the vertex
polyhedron is a Johnson
solid, the tridiminished icosahedron. This
means it is still a Uniform polychoron. Because all cells are regular, it is also semiregular. However, and unlike the
polychora represented by the models above, this is neither regular nor derived from a regular
polychoron by rectification. Its symmetry is no longer the symmetry of the 24cell, it is instead a
ionic
diminished icositetrachoric symmetry.
Fig. 19: This is an icosahedralcellfirst projection of a convex uniform polychoron, the Snub 24cell.
Picture by Jason Wu.
How to build: Have a look at the Eusebeia page on the Snub 24cell.
Also
recommended is David
Richter's page on the 24cell and its snub.
Now, on the symmetry of the model in Fig. 19. When we choose the diagonals of the Cuboctahedron, we
break its Octahedral symmetry. In this case, we have only 3fold rotational symmetry around the
triangular faces (not full reflection symmetry), and for each Square face, we have now a rectangular
symmetry, not a full Square symmetry. This is, again, the pyritohedral symmetry. This remains as
the overall symmetry of the model of the Snub 24cell in Fig. 19 even after the transformation of
the irregular icosahedra into Icosahedra.
Step 4: Glue a Icosahedral pyramid (consisting of an
Icosahedral "base" and 20 regular Tetrahedra as "sides", see Fig. 20) by its base to each of the 24
Icosahedral cells of the Snub 24cell. The Icosahedral cells therefore disappear.
Fig. 20: Icosahedral projection of the Icosahedral pyramid. This can be seen as an "apexfirst"
projection, but also as a "basefirst" projection, where the base is the outer Icosahedron. This
pyramid can be equilateral (in 4D) because the Icosahedron's circumradius (red struts) is slightly
smaller than its edge length (blue struts).
The 120 Tetrahedra of the Snub 24cell are now joined by the 24 × 20 Tetrahedra = 480
Tetrahedra of the new 4D "Pyramids", making a total of 600 Tetrahedral cells. Together with the
previous 96 vertices of the Snub 24cell, the new 24 pyramidal apices add to a total of 120
vertices. The former's vertex polyhedra are still identical; in fact they have all become regular:
20 Tetrahedra now meet at each vertex, which means that the vertex polyhedron is an Icosahedron.
This is also the case for the 24 new pyramidal apices!
Since all cells and vertex polyhedra are regular, we have built a new regular polychoron, the 600cell! It has 1200 Triangular faces
and 720 edges with Pentagonal figures. This is the 4D analogue of the Icosahedron; its symmetry,
known as the Hexachosichoric
symmetry, is the 4D analogue of the Icosahedral symmetry. Its Icosahedral projection is shown
in Fig. 21; this is necessarily a "vertexfirst" projection.
Fig. 21: The Icosahedral projection of the 600cell.
Picture by Jason Wu. See also virtual
model.
How to build: see instructions in David Richter's list of Zometool projects.
If you followed attentively the construction of the 600cell, you can check that the 24 pyramidal
apices have the same arrangement as the vertices of a 24cell. Again, the symmetry of the 600cell
implies that there is nothing special about these apices: the 96 vertices of the Snub 24cell can be
decomposed into 4 groups of 24 vertices, each also with the same arrangement as the vertices of a
24cell. This means that we can build a set of five 24cells with the same vertex arrangement of the
600cell. This is a selfdual regular
polychoron compound, a model is shown in Figs. 31a and b.
Given the method of construction, the edges of the 600cell contain those of the Snub 24cell as a
subset, i.e., the latter is a diminished 600cell. Thus, we
still see the edges of the central blue Icosahedra of the Snub 24cell. But apart from that, we can
also see see other important sections highlighted by the edges: blue Dodecahedra and
Icosidodecahedra. The latter is part of the envelope of the projection; it is therefore an
``Equatorial'' polyhedron (which also has its own equatorial polygon, the Decagon). Studying the
model further, you will also see many flattened versions of those polyhedra; these are identical
sections located in 3D planes that are not parallel to ours. This study will make it easier to
understand and build some of the models below.
One of these is the dual of the 600cell; which is necessarily a regular polychoron with the same
symmetry of the 600cell (Hexachosichoric), 600 vertices with Tetrahedral vertex polyhedra, 1200
edges with Triangular figures, 720 Pentagonal faces and 120 Dodecahedral cells: the 120cell. This is the 4D equivalent of the
Dodecahedron. Its Icosahedral projection appears in Fig. 22. Two of the cells of the 120cell appear
undistorted in the centre of projection: an Icosahedral projection of the 120cell is necessarily a
cellfirst projection.
Fig. 22: This is the Icosahedral projection of the 120cell. This projection is the dual of the
projection of the 600cell in Fig. 21.
Picture taken by Jason Wu. See also virtual model.
How to build: see instructions here. The model in the figure is
larger than the set sold by Zometool as the hyperdo by φ, the famous golden ratio.
If you pay attention to the model, you will see that all of these projections of Dodecahedral cells
can already be seen in the model of the 600cell above! The reason for this is that, as we will see
below, that model also represents the Grand 120cell, which like the 120cell, has 120 Dodecahedral
cells with the same orientations!
This ends the list of the regular convex polychora.
***
The models above represent only a small number of the 64 convex uniform
polychora (the 4D analogues of the Archimedean polyhedra). Some examples I have not built are
in David Richter's
list of convex uniform polychora with Pentachoric symmetry and his list of convex uniform polychora
with Hexacosichoric symmetry. Many uniform polychora with the symmetries of the 16cell and
24cell are not Zomable because they require the construction of Octagons  which are forbidden in the Zometool
system.
Among those "Archimedean" polychora there is a third semiregular polychoron. This results from
rectifying the last regular polychoron with Tetrahedral cells, the 600cell. A model of the Rectified 600cell is shown
in Fig. 23. I have not kept it for lack of Zometool parts, but also because the models of the
regular polytopes and their regular compounds already take a lot of space in my office!
Fig. 23: The Icosahedral projection of the Rectified 600cell. This projection is
the rectification of the projection of the 600cell in Fig. 21.
How to build: After studying the perspectiveflattened Icosahedra in the model of the
600cell, have a look at the Eusebeia page on the Rectified 600 cell. Also
recommended is David
Richter's page on the rectified 600cell.
The 600 Tetrahedra of the 600cell are here replaced by their rectifications, which are Octahedra.
In the position of each of the 120 vertices of the 600cell 120 Icosahedra (the vertex polyhedra of
the 600cell) appeared, two of those appear superposed and without distortion at the very centre of
the projection. The vertex polyhedron is the Pentagonal prism. All projections of the Icosahedron
that appear in this model are the projections of the Icosahedron we've seen in the model of the
600cell in Fig. 21.
If you pay close attention to the model, you will be able to see several Archimedean solids in blue,
starting with Icosidodecahedra, these are sections of the Rectified 600cell highlighted by its
edges. Studying the model further, you will be able to see many flattened versions of those. Such
sections are the cells of 14 nonconvex polychora, all of them edge facetings of the Rectified
600cell and therefore represented by the same model. One of them is the Rectified icosahedral
120cell, which has Great dodecahedra and Icosidodecahedra as cells, more about the Icosahedral
120cell below.
These 14 edgefacetings are a subgroup of the 60 facetings of the Rectified 600cell that are
uniform, nonconvex polychora.
Star polychora
We will now present the ten SchläfliHess polychora  the
regular "star" polychora. These are the 4D analogues of the regular star polyhedra. You can see
their Zometool models rotate, increase in size or even explode with this very cool Observable notebook.
Similarly to what happens in 3 dimensions, all regular star polychora must be stellations and
facetings of regular convex polychora because they must also be symmetric around each vertex and
around each cell. This means that the cell "planes" and vertices must have the same arrangements as
those of one or two regular convex polychora. If the cell plane and vertex arrangements are those of
two different convex polychora, then those must be dual to each other because they must have the
same overall symmetry. These can only be the 600cell and / or the 120cell because, as we will
see next, the few regular stellations and facetings of
the simpler regular convex polychora are all regular compounds.
Stellating a polychoron extends its cells along their 3D "planes" until they meet identically
extended cells, while faceting remove parts of the polychoron while preserving its vertex
arrangement. As for polyhedra (and for the same reasons), if polychora A and B are duals, then the
dual of the stellation of A is a faceting of B. It is very important to keep this in mind for
understanding what follows. As for polyhedra, the larger the number of cells/vertices of a
polychoron, the larger the number of its stellations/facetings.
The different types of stellations and facetings and other geometric transformations between the
120cell, 600cell and the regular star polychora are described in detail in Diagrams IIa, b and c
below. All these operations preserve the Hexacosichoric symmetry of the 120cell and 600cell. Some
preliminary conclusions from them:

In Diagram IIb, we see that nine of the regular star polychora are stellations of the 120cell, and
one of them  the Grand 600cell  is a stellation of the 600cell, with 600 Tetrahedra. This latter
object is therefore the 4D analog of the Great Icosahedron, which it has as a vertex polyhedron.

The dual of each regular star polychoron is also a regular star polychoron; this implies that nine
of them are facetings of the 600cell and that the dual of the Grand 600cell  the Great grand
stellated 120cell  is a faceting of the 120cell (see Diagram IIc). This makes it an analog of the
Great stellated dodecahedron, which it has as cells.

In Diagram IIc, we can see that three facetings of the 600cell  the Icosahedral 120cell (the cells are the
Icosahedral sections visible in the model of the 600cell in Fig. 21, see virtual model), the Grand 120cell (the cells are the
Dodecahedral sections visible in the model of the 600cell, see virtual model) and the Great 120cell (the cells are Great
dodecahedra, which have the same edges as the aforementioned Icosahedra, see virtual model)  share its edge arrangement. This
means that they are share the Zometool representation of the 600cell in Fig. 21! This makes them
analogues of the Great dodecahedron, which has the edge arrangement of the Icosahedron.
We now present Icosahedral projections of the 7 remaining regular star polychora. Figs. 24a, b, 25b
and b represent 6 different facetings of the 600cell.
Fig. 24a: This model represents the Icosahedral projections of two regular star polychora which
share the same edge arrangement, the Small stellated 120cell
(henceforth "Stellated 120cell" for short, see virtual model) and the Great grand 120cell (see virtual model).
Fig. 24b: This is how it looks in my office.
Picture by Jason Wu.
How to build: see instructions in David Richter's
dedicated page.
Fig. 25a: This model represents the Icosahedral projections of four regular star polychora that
share the same edge arrangement: The Grand 600cell (see virtual model), the Great stellated 120cell (virtual model), the Grand stellated 120cell (virtual model) and the Great icosahedral 120cell (virtual model).
Fig. 25b: This is how it looks in my office.
Picture by Jason Wu.
How to build: Make a model of the 600cell, but extend all internal edges: there can be no
broken lines. Then build a new, outer set of vertices of the 600cell, but increased in size by the
Golden ratio, (only a few of these will suffice). For each new outer vertex, see what edges of the
internal model point towards it, then extend those edges to the outer vertex. Do this for a set of
neighboring vertices, which will allow you to work out the intersections of those edges. Once this
is done, apply the same pattern to all outer vertices using Icosahedral symmetry, while removing all
the edges of the outer 600cell that were holding the outer vertices in place. Look here for the justification of this method of
construction.
Note that, as in the previous model, there are several intersecting blue edges. Because I wanted to
make this a permanent addition to my office, I cut the necessary number of blue struts with pliers.
See also David
Richter's page on this projection.
These Icosahedral projections necessarily have the same vertex arrangement as the Icosahedral
projection of the 600cell (Fig. 21). The difference is that for the model in Figs. 24a and b, the
false vertices are those of the Icosahedral projection of the 120cell (Fig. 22); for the model in
Figs. 25a and b they have the same arrangement of a φ smaller 600cell, at the same scale as the
model in Fig. 21.
The models highlight the equatorial polyhedra very well in blue. For the Stellated 120cell, this is
the Dodecadodecahedron, which has the Hexagon as an equatorial polygon. These hexagons are also the
equatorial polygons of 24cells, in a compound of twentyfive of them (see Fig. 33)! For the Grand
600cell, this is the Great icosidodecahedron, which has a Decagram as an equatorial polygon. As
mentioned above, the equatorial polyhedron of the 600cell is the Icosidodecahedron, therefore the
equatorial polyhedra of its facetings must be facetings of the Icosidodecahedron. Still, it is an
interesting fact that all aforementioned rectifications of regular
polyhedra occur as equatorial polyhedra of regular polychora! All of them have themselves
equatorial polygons that are very useful for calculating the metric properties of these polychora.
In Figs. 26a and b, we present the last of the regular star polychora, the most complex of the
16 regular polychora, the final stellation of the 120cell: The Great grand stellated
120cell.
Fig. 26a: Icosahedral projection of the Great grand stellated
120cell, here seen from a 5fold symmetry axis.
Fig. 26b: Same model as in the Figure above, here seen from a 3fold symmetry axis. For many more
perspectives, see a virtual model.
How to build: Make a smaller version of the Stellated 120cell (in Fig. 24a), but with all
internal edges extended  there can be no broken lines. Then identify the equivalent vertices of the
largest possible Dodecahedron in the model, i.e., the outermost balls along the 3fold symmetry
axes. These are the vertices of the central Dodecahedron of a (large) 120cell. Now, based on this,
build a few of the remaining "outer" vertices of that 120cell, at an appropriate scale. Then, from
each of those vertices, see what edges in the inner model point towards it, and extend those edges
until they reach that vertex. Doing this for a set of neighboring vertices will then allow you to
figure out the network of edge intersections. Once this is done, repeat for all vertices of the
120cell using Icosahedral symmetry, while removing all edges of the large 120cell that were
holding the first set of outer vertices in place. Look here for the justification of this method of
construction.
This model can use 20 Y3 struts. These will be directly aligned with the 3fold symmetry axis shown
in this Figure. This saves 40 regularsized struts (Y2 and Y1) and 20 balls and slightly improves
the appearance of the model. This model suffers even more from the blue strut intersections that
appear in the previous two models. In this model, I omitted some of those intersections.
See also David Richter's page on this
projection.
With this model, the list of regular polychora is complete!
We now discuss the geometric relations between the 120cell, 600cell and the regular star
polychora. Instead of 6 polyhedra shown in Diagrams Ia and b, we now have 12 polychora, in Diagrams
IIa, b and c we follow Conway, Burgiel and GoodmanStrauss (see references) and arrange them as the vertices of an imaginary
Cuboctahedron, which as noticed above has 12 vertices and is the only Archimedean polyhedron that is
radially equilateral. This vertex arrangement is, together with the central vertex, part of an optimal sphere packing lattice.
Diagram IIa
As in those earlier diagrams, the vertical scale indicates the density.

Duality. This operation is represented by the twosided yellow arrows. Since it preserves the
density, its arrows are strictly horizontal, as in Diagram Ia. Note that the Great 120cell and the
Grand stellated 120cell are selfdual.

Isomorphism. The red lines that go through the center of the arrangement change a Pentagon
into a Pentagram, or viceversa, in any polygonal element of the polychora at both ends of the line
(these elements are either the cell's faces, the cell's vertex figures, or the edge figures). Thus
the polychora connected by these red lines are isomorphic; this means that their configuration matrices are
identical. An example of this is that the equatorial polyhedra of the 600cell and Grand 600cell
(Icosidodecahedron and Great icosidodecahedron) are, as remarked in the polyhedron page, isomorphic.
Diagram IIb
With this Cuboctahedral arrangement, many other geometric relations between these polychora can be
represented as sets of parallel lines, as in Diagram Ib. In Diagram IIb we show the four stellation
operations that occur among regular polychora; these give them their names.
 Edge stellation. This operation is represented by the dark blue arrows. It has the same
meaning as in 3D, i.e., the Pentagonal faces are stellated into Pentagrams. This operation
preserves the edge
figure. As an example, the Stellated 120cell is formed by stellating each of the 120
Dodecahedra of the 120cell into a Stellated dodecahedron (Fig. 5). The latter has only 12 vertices
(one for each face of the Dodecahedron) arranged as in the Icosahedron. Likewise, the Stellated
120cell has only 120 vertices  one for each cell of the 120cell, arranged as in the 600cell. The
vertex figure of the Stellated dodecahedron cells is a Pentagon; thus the vertex polyhedron of the
Stellated 120cell is a regular polyhedron with Pentagonal faces  the Dodecahedron.
 Greatening. This operation is represented by the light blue arrows; as in 3D this
means replacing a face by a larger dual polygon (see Fig. 11a) in the same 2D plane. Applying this
operation to the 120cell leads to the Great 120cell, where each Dodecahedron of the 120cell is
greatened to a Great dodecahedron (Fig. 5). Edgestellating the Great 120cell, or greatening the
Stellated 120cell, we obtain the Great stellated 120cell, where each Dodecahedron of the 120cell
is expanded into a Great stellated dodecahedron. The pattern of cell stellation in the top blue
circuit is exactly the one shown in Fig. 11b. Greatening the Icosahedral 120cell means that the
cells of the resulting Great icosahedral 120cell are the only regular stellation of the
Icosahedron, i.e., Great icosahedra.
 When greatening (above), the faces are replaced by larger dual polygons in the same plane, which
are of the same type (see Fig. 11a). When expanding cells within the same 3D plane we can, apart
from the polyhedral transformations of the cells listed above, either expand to larger cells of the
same type and orientation or larger dual cells, not both:
 Aggrandizing. The first operation is depicted by the light green arrows. From this, we
see that the cell types in the lower blue circuit are a repeat of the cell types in the top blue
circuit we just described. We also see from this that the Grand 600cell has cells like those of the
600cell, Tetrahedra.
 Dual aggrandizing. The second operation is depicted by the dark green arrows. Thus the
Icosahedral 120cell (the dual of the Stellated 120cell) has 120 Icosahedra as cells.
Even if the cells are selfdual (Tetrahedra), the result of these operations is in principle
different, since a Tetrahedron and its dual have different orientations.
Diagram IIc
In Diagram IIc we show the four faceting operations that occur among regular polychora. These
operations can be seen as the duals of the stellation operations in Diagram IIb, i.e., they are
like a mirror image of the latter. We now describe them individually.
 Face faceting. This operation, represented by the solid red and yellow lines, is
equivalent to: finding dual + edge stellating + finding dual. Since the edge stellation preserves
the edge directions and their edge figures, the dual faceting operation preserves their
perpendicular face arrangements;
which means that the edge arrangement is also preserved. This operation transforms Pentagonal edge
figures into Pentagrams, a consequence of the fact that the dual operation transforms Pentagonal
faces into Pentagrams.
 Edge faceting. This operation, represented by the dashed lines, is equivalent to: finding
dual + greatening + finding dual. Since greatening replaces the faces by larger duals in the same
planes, the dual faceting operation preserves the edge arrangement (not only the edge directions,
but also where they end, since the vertex arrangement is also preserved in any faceting operation)
but replaces the edge figures by smaller duals.
These two operations facet the vertex polyhedra, a consequence of the fact that their dual
operations stellate the cells. Thus, since the 600cell has an Icosahedron as a vertex polyhedron,
the other 3 polychora in the red circuit have as their vertex polyhedra the 3 facetings of the
Icosahedron shown in Fig. 11c: The Icosahedral 120cell has the Great dodecahedron as vertex
polyhedron, the Great 120cell has the Stellated dodecahedron as vertex polyhedron, and the Grand
120cell has the Great icosahedron as vertex polyhedron. Since the Stellated 120cell has the
Dodecahedron as vertex polyhedron, its regular faceting, the Great grand 120cell necessarily has
the only regular faceting of the Dodecahedron, the Great stellated dodecahedron, as a vertex
polyhedron (see Fig. 12c).
 The two following faceting operations are the duals of the two aggrandizing operations.
 Vertex faceting. This operation, represented by the solid blue lines, is
equivalent to: finding dual + aggrandizing + finding dual. Since
aggrandizing preserves the cell type but enlarges it, the dual faceting operation preserves the type
of vertex polyhedron, but reduces its angular size. The light blue lines show that each polychoron
in the red family has a faceting in the yellow family with the same type of vertex polyhedron as
those we described above. The dark blue line shows that the only faceting of the 120cell, the Great
grand stellated 120cell, also shares the former's vertex polyhedron, the Tetrahedron.
 Dual vertex faceting.
This operation, represented by the solid brown lines, is equivalent to: finding dual + dual aggrandizing + finding dual. Since
dual aggrandizing preserves the cell plane but replaces the cells by larger dual cells in the same
3D plane, the dual faceting operation transforms the vertex polyhedron into its dual, while
reducing its angular size.
Importance of Diagram IIc for Zometool models:
Since Zometool models represent vertex and edge arrangements, the first two operations preserve
the Zometool representations!
Indeed, the 4 polychora in the red circuit are represented by the model of the
600cell (Fig. 21) and the 4 polychora in the yellow circuit are represented by the model of the
Grand 600cell (Figs. 25a and b). The 2 polychora in the dashed brown line are represented by the
model of the Stellated 120cell (Figs. 24a and b). The Zometool models of the 120cell (Fig. 22) and
the Great grand stellated 120cell (Figs 26a and b) represent only those polychora.
In the Figure below, we show a Zometool representation of Diagrams IIa, b and c.
Fig. 27: Zometool representation of diagrams IIa, b and c.
The black balls  the vertices of the outer Cuboctahedron  represent the 120cell,
600cell and the 10 regular star polychora, the struts represent the geometrical transformations
discussed above. Note how the edge stellation and its dual, the face faceting, are
special, being the only transformations represented by the blue struts. All others, including
duality, can be represented by green struts and include special instances going through the centre
of the figure, the isomorphisms.
Challenge to the reader: As mentioned above, the Cuboctahedron has Hexagons as "equatorial"
polygons. How many? In Diagrams IIa, b and c, which equatorial Hexagon touches polychora where the
cells match the polyhedra touched by the Hexagon in Diagrams Ia and b? Do the geometric
transformations along that Hexagon match those in Diagrams Ia and b?
Clue: There is a type of stellation that exists only in that Hexagon, but it does not
correspond to a stellation in Diagrams Ia and b, but to another transformation. Which one?
***
Currently we know of 2188
uniform polychora. It is not known if the list is complete or not, but it likely isn't, because
hundreds of them have been found since 2020. Clearly, the vast majority of these are neither
regular nor convex: they are the uniform ``star'' polychora.
Some of these are edge facetings of polychora described above, they are therefore represented by the
same Zometool models. As an example, the aforementioned edge facetings of the Rectified 600cell are
all represented by the model of the Rectified 600cell in Fig. 23.
The only faceting of the 16cell, the Tesseractihemioctachoron, is
an edge faceting; so it shares the Zometool representations of the 16cell. This
polychoron is constructed with the 4 equatorial Octahedra of the 16 cell plus 8 of its 16
Tetrahedra. Since all cells are regular, this is a semiregular nonconvex polychoron. It is the
4dimensional analog of the Tetrahemihexahedron, which it has as
vertex polyhedron; which is necessarily a faceting of the Octahedral vertex figure of the 16cell.
The Dodecahedral vertex polyhedron of the Stellated 120cell (which, as we saw in Diagram IIc, is
unique among all regular polychora) can be faceted in dozens of ways, yielding dozens of uniform edge facetings of the
Stellated 120cell. In Diagram IIc, we used its only regular faceting, the Great stellated
dodecahedron, to obtain the only regular faceting of the Stellated 120cell, the Great grand
120cell. If we use instead the ditrigonal polyhedra in Fig. 7, we obtain three additional
semiregular polychora:
 With the Small ditrigonal
icosidodecahedron we obtain the Small
ditrigonal 120600cell, which has 120 Great icosahedra and 600 Tetrahedra as cells (this name
is consistent with those of the quasiregular polyhedra, which indicate the number of the two types
of faces separately). These Great icosahedra have the same edges as the Stellated dodecahedra of the
Stellated 120cell.
 With the Ditrigonal dodecadodecahedron
we obtain the Ditrigonal
120120cell, which has 120 Great icosahedra and 120 Icosahedra as cells. These Icosahedra can
be seen when we
build the model.
 Finally, with the Great ditrigonal
icosidodecahedron we obtain the Great
ditrigonal 120600cell, which has 120 Icosahedra and 600 Tetrahedra as cells.
In addition to sharing the vertex and edge arrangement of the Stellated 120cell (and thus
sharing the same Zometool representation), these three
polychora share among themselves an identical face
arrangement, a consequence of the fact that their vertex polyhedra, the ditrigonal polyhedra, share
the same edge arrangement.
Being edge facetings of regular polychora, the Tesseractihemioctachoron and the three ditrigonal
polychora are isogonal and isotoxal. Furthermore, their vertex polyhedra are also isotoxal, which
each edge separating the two types of faces; this implies that these four polychora are also isohedral,
whith each face (Triangles in all four cases) separating the two kinds of cells, all of which are
regular. No other semiregular polychora come this close to full regularity.
Additionally, two of the facetings of this Dodecahedral vertex polyhedron are regular compounds: the
Compound of five cubes and the Compound of ten tetrahedra. This is important: as we will see
later,
this implies that some regular polychoron compounds are Zomable.
Main polytope page.