Stewart notation

The Stewart notation is used to describe Stewart toroid s. It was developed by Bonnie Stewart in the book Adventures among the Toroids. It abbreviates each polyhedron to a letter and number, and shows how groups of them are connected to or removed from one another.

Convex polyhedral units
The series of polygonal prism s, polygonal antiprism s, and polygonal pyramid s each have a letter associated with them. The polygon base of a polyhedron determines its number. Three Platonic solids can be represented as members of these series. The two that can't have new letters set aside for them. The letters used for Archimedean solids loosely follow some rules: truncation of a polyhedron with 3 edges at a vertex = T, rectification = B, cantellation/expansion = E, omnitruncation = K. The numbers used are: 3 for polyhedra with tetrahedral symmetry, 4 for cubic symmetry, and 5 for dodecahedral symmetry.

These letters act like a weaker version of the Conway polyhedron notation's operators. Without an operator for bitruncation, Stewart notation takes advantage of the fact that the truncated octahedron is the omnitruncated tetrahedron, and sets aside another new letter for the truncated icosahedron.

The two snub Archimedean solids are barely mentioned in Adventures. Johnson solids are mostly notated by a "J" followed by their index in Norman Johnson's enumeration. However, the simpler dome-like ones that cannot be decomposed into other Johnson solids - the polygonal cupola e and the pentagonal rotunda, as well as the aforementioned pyramids - get their own letters.

Many Johnson solids go unused. Among the ones that are used, if they can be built with augmentation s and elongation s, they are usually referred to as a stack of smaller units. For example, the elongated pentagonal orthobirotunda J42 is usually referred to as R5P10R5. This informs us of its structure, and keeps us from having to memorize or look up the indices of all 92 Johnson solids.

Other polyhedral units
Since Stewart toroids are by definition nonconvex, there is no reason to impose a restriction of convexity upon the polyhedra they're made of (except for the excavated-from polyhedron in a quasi-convex toroid). Some nonconvex polyhedra occurring relatively frequently in Stewart's investigations were given names in order to more easily refer to them.

Stacking polyhedra
A common feature in Stewart toroids is the use of many polyhedral units outer-blended together. When these occur in a linear fashion (i.e. not forming branches or loops), the Stewart notation represents them by writing in a sequence the abbreviations of the involved polyhedra. A simple example of this, as stated before, is how the elongated pentagonal orthobirotunda can be referred to as R5P10R5 which more clearly describes its structure. But a stack of polyhedra does not need to be convex, and has no constraints on its length.

Especially repetitive linear stacks may be denoted with a numerical exponent after the repeated unit, like 4. If these are made of prisms, they will contain many coplanar adjacent faces, so further modifications may be made to satisfy (A). These can be as short as 2 or the pentagonal orthobicupola.

If a stack of polyhedra loops back on itself, a description of it as a sequence would fall short. The Stewart notation represents such loops of polyhedra as a list of counts of each unit in the loop. For example, a ring of heptagonal prisms, with the (A) condition met by inserting hexagonal prisms in between (each one rotated 90° in the plane of its blended-away squares), is referred to as.

If there is a polyhedron that is in the center of a stack, it may be surrounded by parentheses, like in.

If a polyhedron is in the center of an arrangement that many congruent stacks branch off of, write the number of those stacks, then write them down, then write the central polyhedron (where all the stacks join) in parentheses. An example of this is. Large stacks don't need to be surrounded by parentheses for clarity.

Tunnels: genus-changing
The operation of excavating one polyhedron from another larger one in a way that changes the genus of the latter (i.e. the excavation goes all the way through the larger polyhedron) is represented by a forward-slash or division sign $$/$$. On its left side is written the larger polyhedron that has a tunnel dug through it, and on its right side is written the shape of the polyhedral tunnel that is being dug out of the larger polyhedron.

Stewart toroids with higher symmetry and higher genus can have many copies of the same tunnel going from the outside to an inner, central polyhedron. In this case, the syntax for central polyhedra with many congruent stacks branching off is used on the right side of the $$/$$.

Augmentations and excavations: non-genus-changing
For augmentations and excavations of polyhedra to a "base" polyhedron that don't change the genus of the base polyhedron, the name of the augmented or excavated polyhedron is written after the base polyhedron, along with an exponent that contains the number of instances of it on the base polyhedron and either the letter "A" (for augmentation) or "E" (for excavation). It is also customary to precede the number of an excavation with "-".

For example, a triangular prism of edge length 4 made out of 64 unit-edge-length triangular prisms can be tunnelled along its axis by four triangular prisms. This can be referred to as 4, but both the original polyhedron and the tunnel will have coplanar adjacent faces. Eight square pyramids can be excavated from each "square face" of the large triangular prism, and six tetrahedra from each "triangular face." These inward-facing tetrahedra surround the ends of the tunnel closely, so we can't augment every other triangular prism in the tunnel, but we can augment every face of the middle two. This gives us -24E-12E46A.

Also, Stewart's T polyhedron is J91'J63A, W is -3E, and W '' is -4E.

The fact that this part of the notation doesn't specify exactly which faces or orientations the augmentations or excavations occur on could be called a weakness. However, this probably wasn't a big concern for Stewart, as this part of the notation was mostly only used to keep adjacent faces from being coplanar. As long as one saw that augmentations or excavations were demanded by a name or problem, they could look into how to lay out those modifications in a way that kept adjacent faces from being coplanar (and that avoided self-intersections), and any valid solution they arrived at would be fine by Stewart.