Polyhedra with the Zometool

"Thus, the chief reason for studying regular polyhedra is still the same as in the time of the Pythagoreans, namely, that their symmetrical shapes appeal to one's artistic sense."
H. S. M. Coxeter, in the preface to "Regular Polytopes".

In Figs. 3.3a, b and c, we have seen three models of polyhedra built with the Zometool. Such edge models display only the vertex and edge arrangements; unlike paper models they cannot display face arrangements. A consequence of this is that the Icosahedron and the Great dodecahedron, which share the same edge arrangements, are represented by the same edge model. The same happens for the Stellated dodecahedron and the Great icosahedron. The ditrigonal polyhedra in Fig. 3.7 and the Compound of five cubes in Fig. 3.8b share the same edge arrangement and thus the same edge model.

However, this can also be an advantage: because they don't display the faces, they allow the display of internal structure. For the models that follow that is absolutely necessary. As an example, the first two models below represent the five Platonic solids. However, they are not represented in isolation (something that would merely repeat the paper models above), but instead in an arrangement that illustrates many interesting geometric relations between them, in a very elegant and economic way. Clearly, such a model cannot be made with paper. The first model in particular was the first kit of the Zometool I purchased; it was especially useful for me to familiarise myself with the interesting properties of the system.

The Tetrahedron and Octahedron are built with G struts; the other regular polyhedra are built with B struts. These colours are the same for their rectifications: for instance, the Octahedron is built with G struts like the Tetrahedron, the Cuboctahedron is built with G struts like the Octahedron (not the Cube); the Icosidodecahedron is built with B struts like the Icosahedron or Dodecahedron. The reason for this can be seen in Fig. 2.4d, where each Pentagonal face of the Dodecahedron is rectified into a smaller Pentagon.

Most models in this page (but not the following pages on polychora) have G or B struts with non-native colours. These can be purchased from the Zometool company on demand, but they are slightly more expensive than struts in native colours. They are used here to help distinguish the regular polyhedra from each other. The first model was designed by John H. Conway. The following models were designed by myself, the stellation nets of the stellation models and the inside-out step-by-step models showing the construction of the faceting models should make the construction clear.

Fundamental models

 We now present a set of models that illustrate some fundamental properties of polyhedra and of the Zomeool.

The model in Fig. 4.1a includes models of the five Platonic solids. This is a kit sold by the Zometool company with the name ``Kepler's Obsession''. This was designed by John H. Conway, who called it his ``Cosmogram''. It is not Kepler's polyhedral model of the Solar System published in his Mysterium Cosmographicum, but it bears some resemblance, hence its name. Fig. 4.1b shows a larger version I have built.


Fig. 4.1a: Myself holding a beautiful geometrical model sold by the Zometool company as Kepler's Obsession. Photo by Aris Noutsos.


Fig. 4.1b: This is a larger version of the model above, which I have made using some of the specially coloured struts from the model above and other rare parts of the Zometool*.


The innermost polyhedron, the Icosahedron (represented in the model in Fig. 4.1b by the red B1 struts) is the dual of the outermost polyhedron, the Dodecahedron (dark blue B2 struts). The second-innermost polyhedron, the Octahedron (light blue HG1 and HG2 struts) is the dual of the second outermost one, the Cube (yellow B3 struts). The middle polyhedron, the Tetrahedron (green HG3 struts) is self-dual. The model is put together in such a way that each vertex of a polyhedron lines up with the face of its dual, and vice-versa; furthermore, the edges of dual polyhedra are perpendicular to each other. An interesting implication of this is that the whole model is, in a way, also self-dual! Also, Thus, when the vertex of an inner polyhedron A touches and edge of inner polyhedron B, the edge of the dual of B is a diagonal of the face of the dual of A. The construction is possible because the number of vertices of the Octahedron (6) and Icosahedron (12) are the same as the number of edges of the Tetrahedron (6) and Octahedron (12) - by duality, the 6 and 12 edges of the Tetrahedron and Cube correspond to the 6 and 12 faces of the Cube and Dodecahedron.

Challenge to the reader: The numerical coincidence of vertices of the Octahedron and edges of the Tetrahedron is easily explained: the Octahedron is the rectification of the Tetrahedron (see previous page). However, is there any special reason why the number of vertices of the Icosahedron (12) has to be the same as the number of edges of the Octahedron? (An answer is provided in the caption of Fig. 5.7).

We can also see that: Thus, the facial plane coincidences of the inner polyhedra - all of them with Triangular faces - are the duals of the vertex coincidences of the outer polyhedra - all of them with Triangular vertex figures. These vertex coincidences are explored in Figs. 4.3a, b and c.

Many other things can be seen in the model. For instance, if we extend the edges of the inner Icosahedron, they meet two vertices of the outer Dodecahedron. The edge arrangement would then be that of the Great stellated dodecahedron. Also, all the edges of the inner Octahedron do this as well, but only in one direction! It is not clear to me why this happens!

The models above have no mirror symmetry; only a type of symmetry called chiral Tetrahedral symmetry. To achieve mirror symmetry, we would need to add a second Tetrahedron, the dual to the one in the model, making a Stella Octangula configuration (Fig. 3.8a and 4.5a, b below). Then, we'd have a complete stellation of the Octahedron and a complete faceting of the Cube in this model. The resulting model would then have an overall pyritohedral symmetry.

Now, if we rotate that model four times around an axis going through the centre of two opposite Pentagonal faces of the Dodecahedron, and superpose these to the original - a process we will call here "Quintuplication" - we find that the inner Icosahedron stays unchanged, the Octahedron becomes a Compound of five octahedra (illustrating how it results from a stellation of the Icosahedron), the Stella Octangula becomes a Compound of ten tetrahedra (illustrating how the latter compound results from a stellation of the Icosahedron, being self-dual it is also a faceting of the Dodecahedron), the Cube becomes a Compound of 5 Cubes (illustrating how it results from the faceting of a Dodecahedron, this is the dual of the inner Compound of five octahedra) and the outermost Dodecahedron (the dual of the innermost Icosahedron) stays the same. All of these facetings and stellations are shown in the models below.

Thus, a 5-fold rotation of an object with pyritohedral symmetry around a carefully chosen axis transforms it into an object with full Icosahedral symmetry! This is a fact we will be using often for what follows.

* Some words on this. The model in Fig. 4.1b represents an improvement compared to the model in Fig. 4.1a because:
a) each polyhedron is now represented by a different colour: in Fig. 4.1a, the Octahedron and the Tetrahedron are in the same colour, blue-green. That error (later fixed by the Zometool company) was useful: I used the 12 extremely rare light blue HG2 struts of the Tetrahedron in 4.1a to build the larger Octahedron in 4.1b.
b) The edges of this Octahedron are no longer dominated by the twists of the HG parts.
The inner Icosahedron in 4.1b is made with the red B1 struts of the outer Dodecahedron in 4.1a. The outer Dodecahedron in 4.1b is made using commonplace B2 struts. For the Cube in 4.1b, I've had for a long time the rare yellow B3 equivalents of the yellow B2 struts in 4.1a. What finally made the model in 4.1b possible was the surprise inclusion of a set of extremely rare green HG3 struts in a large lot of the Zometool parts I have purchased in 2019, these are used to make the Tetrahedron.
I like the colour scheme of the model in Fig. 4.1b very much. The colours of the dual pairs are opposed: If we associate these colours with a sequence in the Rainbow (red, yellow, green, blue-green, dark blue/violet), then the inner/outer solids have the extremes of the sequence, the middle solid has the middle colour (green) and the others have the remaining, not-so-extreme colours. Relative to Fig. 4.1a, the colours of the inner/outer pair were exchanged, something that increases the contrast between polyhedra and makes it easier to distinguish them.

***

I now show several models from which the metric properties of the regular polyhedra, and even their vertex coordinates, can be easily inferred. These don't include the Octahedron, for which these properties can be inferred from its equatorial Square.


Fig. 4.2: Three orthogonal Golden triangles, in a configuration with pyritohedric symmetry. The vertices are those of the Icosahedron.


The first is a set of three Golden rectanges (prepresented by the B1 and B2 struts), together with their diagonals, represented by the red R1 struts, as shown in Fig. 2.2d. These rectangles are arranged along perpendicular planes in such a way that the overall arrangement has pyritohedral symmetry. The object has a total of 4 vertices for each of the three rectangles, thus a total of 12 vertices, and a total of 12 R1 struts. These connect to the 12 pentagonal holes of the central ball. The rectangles are arranged to each other as Borromean rings.

It should be clear that the R1 struts are arranged with Icosahedral symmetry, and that the vertices they connect to are those of the Icosahedron. The B1 struts represent the edge of an Icosahedron (or its faceting, the Great dodecahedron) of length ℓ = 1. R0 is the length of the red strut, √ ((5 + √5) / 8). R1 is easy to compute: it is half the larger side of the Golden rectangles, φ/2.

The B2 struts represent the edge of an Stellated dodecahedron (or its faceting, the Great icosahedron), thus in this case ℓ = φ. Again, R0 is the length of the red strut, √ ((5 + √5) / 8). R1 is easy to compute: it is half the smaller side of the Golden rectangles, 1/2.

If we quintuplicate the model in Fig. 4.2 and remove the red struts, we will have an Icosahedron and a Great Icosahedron, with the same 12 vertices. This is shown in Fig. 4.6c.

The following Figures are inspired by the vertex coincidences of the polyhedra with Triangular vertex figures in Figs. 4.1a and b, the Tetrahedron, Cube and Dodecahedron This means that, for all these figures, the distance to the centre is that of a Y strut. This has to be because the latter defines axes of 3-fold symmetry, which are consistent with the vertex figures of these polyhedra.


Fig. 4.3a: A Cube built with B2 struts. Its diagonals are represented by Y1 struts.


Fig. 4.3b: The diagonals of the faces of the preceding Cube are, if chosen correctly, the faces of a Tetrahedron, here represented by G2 struts. The distance of its vertices to the centre is also the length of a Y2 strut.


Fig. 4.3c: To the model in Fig. 4.3b, we add three Long yellow rectangles, in a configuration with pyritohedric symmetry. The vertices of these rectangles and the Cube are those of the Dodecahedron.


In what follows, we will re-define our distances, for ease of calculation. Now our unit of measurement is the length of a B2 strut. I did this because the figures are clearer at this large scale.

In Fig. 4.3a, we represent a Cube with its diagonal struts. For a Cube with ℓ = 1, the diagonal is given by √(12 + 12 + 12) = √3. Thus, R0 = √3/2, which is the length of a yellow strut (as seen in Figs. 2.2b and 2.2e, in Figs. 4.3a, b and c this is represented by a Y2 strut). The distance of centres of the faces from the centre of the Cube are easy to deduce from the shape: 1 /2 of an edge length.

In Fig. 4.3b, we see again the Tetrahedron incribed in the Cube. We see that the Y strut is also the distance from the vertex to the centre (R0) of the Tetrahedron, only that the latter has an edge that is √2 times larger than that of the Cube. We also see that the centre of the Tetrahedron is passing through the centres of the Cubic faces, thus R1 = 1/2. We can also see here halves of the Yellow rectangles in Fig. 2.2b, i.e., right angle triangles with blue, green legs and a yellow hypotenuse.

In Fig. 4.3c we add to the previous model three Long yellow rectangles. As we saw in Fig. 2.2e, a Y strut of length √3/2 is also the distance from the vertex to the centre (R0) of these rectangles. These three Long yellow rectangles have the same type of Borromean arrangement and the same pyritohedric symmetry as the three Golden rectangles in Fig. 4.2. As in Fig. 4.2, the number of vertices of these rectangles is that of the Icosahedron (12).

It should be clear that the Y2 struts of the object in Fig. 4.3c connect to all 20 triangular holes of the central ball, and thus have Icosahedral symmetry. Thus, the vertices they connect to are those of the Dodecahedron. From this, we see that the number of vertices of the Dodecahedron has to be the number of vertices of the Icosahedron + the number of vertices of the Cube (and thus the number of faces of the Icosahedron is the same as the number of faces of the Dodecahedron + of the Octahedron).

The B1 struts of the Long yellow rectangles represent the edge of the Dodecahedron, of length ℓ = 1/φ relative to the edge of the Cube (as was already apparent in Figs. 4.1a, b). R1 is easy to compute: it is half the larger side of the Long yellow rectangle, φ/2.

The B3 struts of the Long yellow rectangles represent the edge of the Great stellated dodecahedron, of length φ relative to the edge of the Cube. R1 is easy to compute: it is half the smaller side of the Long yellow rectangle, 1 /(2 φ).

If we quintuplicate the model in Fig. 4.3c and remove the yellow struts, we will have a Compound of five Cubes, a Compound of five tetrahedra, a Dodecahedron and a Great stellated dodecahedron, all with the same 20 vertices. This object is shown in Fig. 4.8.

***

We now show two rectangular cuboids that summarise many of the metric properties of the polygons and show how they reappear in the quasi-regular solids. The first has two Squares (Fig. 2.2a) and four Yellow rectangles (Fig. 2.2b) as faces, this I call the Yellow prism. The diagonals have length √(1 + 1 + 2) = 2, thus representable with two struts of length 1.


Fig. 4.4a: A Yellow prism has sides 1, 1 and √2. The diagonal is 2, thus representable with two struts of length 1 (in this model, represented by the HG2 struts). The faces are two pairs of Yellow rectangles and a pair of Squares.


As the figure shows, not only is this cuboid Zomable, but so are its unit diagonals. When all diagonals are built, they meet at the centre, from which we see clearly that the angles between the diagonals are 60, 90 and 120 degrees, the values of α of the Hexagon, Square and Triangle and the values of β of the Triangle, Square and Hexagon. The edges of the cuboid, or the diagonals of its faces, correpond to the sides of these Polygons. The edges of the cuboid or its diagonals correspond to the chords of these angles.

The second cuboid has two Golden rectangles (Fig. 2.2d), two Long yellow rectangles (Fig. 2.2e) and two smaller Golden rectangles as faces. I call this a ``Golden cuboid", with edges 1/φ, 1 and φ. The diagonal is √(1/φ2 + 1 + φ2) = 2 (see eq. g), which means they are also represented by two struts of length 1. The volume is φ × 1 × 1/φ = 1 as well.


Fig. 4.4b: A Golden cuboid has sides 1/φ, 1 and φ (here a B2 strut represents unity, the B1 and B3 struts represent 1/φ and φ). The diagonal is 2, thus representable with two struts of length 1. The faces are two pairs of Golden rectangles (one φ times larger than the other) and a pair of Long yellow rectangles.


As the figure shows, not only is this cuboid Zomable, but so are its unit diagonals. Furthermore, the angles between diagonals seen from the centre are 36, 60, 72, 108, 120 and 144 degrees, the values of α for the Decagon, Hexagon, Pentagon, Decagram, Triangle and Pentagram. These angles also correspond to the values of β for the Pentagram, Triangle, Decagram, Pentagon, Hexagon and Decagon. The edges of this cuboid and the diagonals correspond to the sides and radials of the even-sided Zomable Polygons, the diagonals of the rectangular faces (Figs. 2.2d, e) correspond to the sides of odd-sided Zomable Polygons. The edges are the chords of the values of β for the odd-sided Polygons, the diagonals of the rectangular faces correspond to the chords of β for the even-sided polygons.

These figures will reappear below in the models of the facetings of the quasi-regular solids.

Stellations and facetings of the Platonic solids

 In what follows, we depict how many of the polyhedra depicted by the paper models above - especially the star polyhedra - relate to the convex polyhedra by stellation and faceting. For this, we will use stellation diagrams; also represented using the Zometool. In the stellations, the real vertices are indicated by the white balls; false vertices (where edges intersect, but don't end) appear in other colours. For the facetings, the only real vertices (of the outer polyhedron, which is the convex hull of all objects further in) will be depicted in black, false vertices appear in other colours. The facial plane coincidences in the models showing stellations correspond to the vertex coincidences in the models showing facetings. Most models with icosahedral symmetry use size 3 struts.

Incidentally (as this was not my initial intention), these models will prove to be quite useful for understanding the polychora and polychoron compounds that will be discussed in the subsequent pages!


Fig. 4.5a: This is the stellation diagram of the Octahedron. The inner Triangle is the face of the Octahedron. The outer dual Triangle, with an edge that is twice as long as that of the inner Triangle, is the face of its only stellation, the Compound of two tetrahedra (Fig. 3.8a). This type of stellation is called a "greatening".


Fig. 4.5b: Expanding all faces of the Octahedron as in the previous Figure, we obtain on the left a model of the Octahedron (light blue) and the Compound of two tetrahedra (green). On the right is the dual of the previous model, showing the Cube (in yellow) and further in, with the same vertex arrangement, the Compound of two tetrahedra (green); this also appears here because it is self-dual.


The models in Fig. 4.5b follow the colour scheme of Fig. 4.1b, and represent the three medial polyhedra in that model. We cannot invert the relations because there are no stellations of the Cube, therefore no facetings of the Octahedron.

The more complex models below continue the same theme of stellations and facetings. One common characteristic is that the inner and outer polyhedra are isomorphic to each other.

We start with the stellations of the Dodecahedron and the facetings of the Icosahedron. Here we can represent all stellations and facetings with relatively simple models!


Fig. 4.6a: This is the stellation diagram of the Dodecahedron; its face is the inner Pentagon. The inner Pentagram, with edge length 2φ +1 = φ3 times that of the inner Pentagon, is the face of the Stellated dodecahedron. The outer Pentagon - a φ + 1 = φ2 larger dual of the inner Pentagon - is the face of the Great dodecahedron, and the outer Pentagram - a φ2 larger dual of the inner Pentagram - is the face of the Great stellated dodecahedron.


Fig. 4.6b: Extending the 12 faces of the Dodecahedron as in Fig. 4.6a, we obtain the four stellations of the Dodecahedron (see Fig. 3.5a, blue rectangle in Diagram Ib).


Fig. 4.6c: In this model, the dual of the model in Fig. 4.6b, we represent the four facetings of the Icosahedron (see Fig. 3.5a and green rectangle in Diagram Ib), which is polyhedron in yellow B2 struts. Also represented, with the same edge arrangement but pentagonal faces, is the Great dodecahedron. Further in, in blue, is the Stellated dodecahedron. Finally, the innermost polyhedron, with the same edge and vertex arrangement of the Stellated dodecahedron but with triangular faces is the Great icosahedron. The model has the same edges as the model in Fig. 4.6b, except for the absence of the outer layer.


Note how in Fig. 4.6c, the vertices are connected to the vertex figures of the four polyhedra by their edges, and how those vertex figures are highlighted by the edges of the full set of polyhedra: for the Icosahedron, it is the yellow Pentagon around each vertex, for the Great Dodecahedron it is the blue Pentagram under each vertex, for the Stellated dodecahedron, it is the yellow Pentagon around the opposite vertex, and for the Great icosahedron it is the blue Pentagram under the opposite vertex. These vertex figures/faces are the duals of the faces/vertex figures in Fig. 4.6b.

We now continue with the stellations of the Icosahedron. We represent only a few of the 22 cases in each class, mostly objects that we have represented as paper models.


Fig. 4.7a: An incomplete stellation diagram of the Icosahedron, representing four of the figures that can be built with a small model in the Zometool. The inner blue Triangle is the face of the Icosahedron, the green Triangle is a face of the Compound of five tetrahedra (see Fig. 3.8a) and the outer φ4 larger dual Triangle is a face of the Great icosahedron (see Fig. 3.5a). An intermediate figure where each face is composed of three disjointed (blue) triangles, touching the face of the Icosahedron at its vertex, is the aforementioned Excavated dodecahedron. A few other stellations were presented already in Fig 3.7, but those are not Zomable.


Fig. 4.7b: Extending the 20 faces of the Icosahedron as shown above, we have the four stellations of the Icosahedron mentioned above.


And now we continue with the facetings of the Dodecahedron. This model is not strictly a dual of the previous one because it also includes the ditrigonal polyhedra.


Fig. 4.8: This model represents facetings of the Dodecahedron, this is the outer polyhedron represented by the white B3 struts, the convex hull that circumscribes all other polyhedra further in that were represented by paper models.


Note how, for each vertex, the blue Triangles (faces of the Small ditrigonal icosidodecahedron) trace the vertex figures of the Dodecahedron and, near the opposite vertex, of the Great stellated dodecahedron, being connected to them by the edges of these Polyhedra. You can also see how the irregular hexagonal vertex figures of the ditrigonal polyhedra are traced below each vertex by the edges of the Dodecahedron, Great stellated dodecahedron and the ditrigonal polyhedra.

We now show the construction process, starting from the polyhedron further in, then adding successive layers.


Fig. 4.8a: At the centre of the previous model, in yellow B1 and B2 struts, is the Great stellated dodecahedron (Fig. 3.5a).


Fig. 4.8b: The green struts represent the Compound of five tetrahedra (Fig. 3.8a). The Compound of ten tetrahedra (Fig. 3.8a) is also a faceting of the Dodecahedron, but it is not Zomable, as it would require two green struts to connect to the same holes.


Fig. 4.8c: The blue B1/B2 struts represent the ditrigonal polyhedra (Fig. 3.7) and the Compound of five cubes (Fig. 3.8b), all of which share the same edge arrangement.

Stellations and facetings of the quasi-regular solids

 In the previous sections, all edges of the regular and partially regular polyhedra and their compounds can be represented with G and B struts, the reason for this is explained by Figs. 4.1a and b, 4.2 and 4.3a, b and c. The Y and R struts have only been used for the radials of the models in the latter figures. The quasi-regular polyhedra shown below can also be represented with G and B struts, the reason is that thay have two types of faces, one of which is a rectification (thus identical) to the faces of a regular polyhedron (see e.g., Fig. 2.4d).

However, this is not the case for the rhombic solids. In Fig. 4.9, we show their rhombic faces and two intersecting diagonals of the compounds of regular solids in Figs. 3.3b and 3.3c, all centred on black balls.


Fig. 4.9, Top: the intersecting edges of the Cube (B1) and Octahedron (G1) in Fig. 3.3b can be inscribed in a Yellow rhombus (see Fig. 2.5a). Bottom: the intersecting edges of the Dodecahedron (B1) and Icosahedron (B2) in Fig. 3.3c can be inscribed in a Golden rhombus (see Fig. 2.5b).


Since the rhombic polyhedra are the convex hulls of these compounds of Platonic solids, their faces are in the same plane as these two intersecting edges, and must be the smallest polygons to fully circumscribe these diagonals. These are, respectively, Yellow and Golden rhombuses, with the same edge colours (Y and R) and in this case dimensions as those rhombuses in Figs. 2.5a and 2.5b. Compare these figures with the Yellow and Golden rectangles in Figs. 2.2b and 2.2d.

We now show models of these rhombic polyhedra (with Y and R struts) and their stellations, and also duals models of these, showing facetings of their duals, the Cuboctahedron and Icosidodecahedron. Before each pair of models, we show, as above, stellation diagrams for the isohedral polyhedra to be stellated (the rhombic solids). We start with a stellation of the Rhombic dodecahedron and a facetings of its dual, the Cuboctahedron.


Fig. 4.10a: This is the stellation diagram of the Rhombic dodecahedron. There are four stellations, with vertices indicated by the connector colours: the rhombic dodecahedron (black balls), first stellation (blue balls), second stellation (yellow balls) and third stellation (green balls).


Fig. 4.10b: Further in is the Rhombic dodecahedron, outside is its first stellation. The latter object is known as "Escher's solid". This particular shape can fill space.
It is composed of three Square bipyramids, with Square bases in blue and yellow struts representing all other edges. I call it the Yellow bipyramid.


Fig. 4.10c: The dual of the model in Fig. 4.10b, the Cuboctahedron and, further in, its first faceting, by three three yellow prisms (Fig. 4.4a).


Note all edges the last figure belong to a set of three yellow prisms (Fig. 4.4a). These yellow prisms are the duals of the three yellow bipyramids in Fig. 4.10b. In the models above, we could show additional stellations of the Rhombic dodecahedron, but its dual facetings of the Cuboctahedron are not Zomable. The diagonals of these three yellow cuboids (shown in Fig. 4.4a) are the Zomable radials of all equatorial Hexagons in this model (see Fig. 2.3c), but they not represented here.

An interesting thing about the dual objects in Figs. 4.10b and c is that they have an identical vertex arrangement. This means, by duality, that they have an identical arrangement of facial planes!

Below, we show a model of the four isotoxal stellations of the rhombic triacontahedron. They can be found very easily since they are represented by different colours. For more on them and instructions on how to build them, and many more stellations of the rhombic triacontahedron, look here.


Fig. 4.11a: Within the inner region of the stellation diagram of the Rhombic triacontahedron (shown in full detail here) we can find its four isotoxal stellations, for which we have built paper models above. The inner Golden rhombus (in red, see also Fig. 2.4b) is one of the 30 faces of the Rhombic triacontahedron itself (Fig. 3.4). The Long yellow rhombus (Fig. 2.4c) is one of the 30 faces of the Medial rhombic triacontahedron (see Fig. 3.6a). The blue Square is one of the 30 faces of the Compound of 5 cubes (Fig. 3.8b). The outer Golden rhombus, which is 2φ + 1 = φ3 times larger than the inner red Rhombus, is one of the 30 faces of the Great rhombic triacontahedron (Fig. 3.6a).


Fig. 4.11b: Extending the 30 faces of the Rhombic triacontahedron as in Fig. 4.11a, we obtain this model.


We now show the dual model, with the four isotoxal facetings of the Icosidodecahedron. In this model we can see that the sides of the Pentagrammic faces of the Dodecadodecahedron (in blue) are the diagonals of Pentagonal faces of the outer Icosidodecahedron that circumscribe them (in yellow), being therefore φ times longer. Similarly, the sides of the Pentagrammic faces of the Great icosidodecahedron (in white) are the diagonals of the Pentagonal faces of the Dodecadodecahedron that circumscribe them, being therefore φ times longer again.


Fig. 4.12: This model, the dual of the model in Fig. 4.11b, shows the four isotoxal facetings of the Icosidodecahedron, which is the outer shape in yellow B3 struts, the convex hull that circumscibes all polyhedra further in.


Note that apart from the edges of the Compound of five octahedra (in green), all edges this figure belong to fifteen Golden cuboids (see Fig. 4.4b) aligned with the 15 axes of 2-fold symmetry of the model: the 1 / φ edges are those of the Icosidodecahedron, the "unit" edges, which correspond to the length of the radials of the Golden cuboid, are those of the Dodecadodecahedron - which is radially equilateral - and the φ edges are those of the Great icosidodecahedron. The vertices of these 15 Golden cuboids are superposed by fours on each of the 30 vertices of the Icosidodecahedron. There are three faces of each of these golden cuboids under each real vertex of the compound, they correspond to the three rectangular vertex figures of the three facetings of the Icosidodecahedron, appearing at increasing depths under each vertex and connected to it by the edges of the respective polyhedra. These vertex figures are the duals of the rhombic faces of the polyhedra in Fig. 4.11b.

The radials of these 15 Golden cuboids (shown in Fig. 4.4b) are the Zomable blue radials of all equatorial Decagons, Hexagons and Decagrams in this model (see Figs. 2.3c, d and 2.8b), but they not represented here.

We now show the construction process, starting from the polyhedron further in, then adding successive layers.


Fig. 4.12a: At the centre of the model is the Great icosidodecahedron, in white B1 and B2 struts.


Important: If you build the model in Fig. 4.12a, you'll notice an interesting fact: At the centre of the model, you will see an Icosidodecahedron with "false" white vertices located directly under the outer real black vertices, with an arrangement that is φ times smaller. This fact is not obvious from the paper model in Fig. 3.6a.

The reason for this is that the edges of the Great icosidodecahedron are obtained by extending the edges of an Icosidodecahedron: the Pentagons of the Icosidodecahedron were stellated into isomorphic Pentagrams with a φ3 times larger side (see Fig. 2.6a) and its equatorial Decagons into isomorphic equatorial Decagrams, also with a φ3 times larger side (see Fig. 2.8b). As we've seen in the study of the sizes of Polygons, the vertices of a Decagram are located right above the vertices of the Decagon they stellate (Fig. 2.8b), with an arrangement that is φ times larger (Fig. 2.8c).

The fact that these are equatorial polygons of the inner Icosidodecahedron and Great icosidodecahedron imply that the vertices of the latter are also located right above those of the former, also with an arrangement φ times larger, as we see in the model. This fact will have important implications for understanding some of the regular star polychora.

Fig. 2.8c can thus be seen as a cut of Fig. 4.12 along its equatorial Decagons. These correspond to the outer Decagon in Fig. 2.8c. The equatorial Decagram of the inscribed Great icosidodecahedron (Fig. 4.12a) is the inscribed Decagram in Fig. 2.8c.


Fig. 4.12b: Just outside the Great icosidodecahedron is the Compound of five octahedra, in green G2 and G3 struts. Note the 3-intersections of some of the G3 struts in the 3-fold symmetry axes, where they have to bend a bit.


Fig. 4.12c: Just above the Compound of five octahedra is the Dodecadodecahedron in blue B1 and B2 struts.


One could also make models with the stellations of the Coboctahedron and Icosahedron and the facetings of the Rhombic dodecahedron and triacontahedron, but none of those stellations is isotoxal.

One last word about the models in this page. They are the closest in the whole site to artistic objects: one can choose which combinations of polyhedra to show, and their sizes and orientations relative to each other, furthermore we have some freedom in the choice of colours. The models in Figs. 4.1a and b are in my opinion elegant works of art. Regarding the other models, although they merely display stellations and facetings, they can also be very pleasing. I am particularly happy with those in Figs. 4.7b and 4.11b. The model in Fig. 4.8c is also quite striking!


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