Step prism

A step prism is an isogonal polychoron whose vertices swirl in correspondence to a star polygon. Similarly to how the n vertices of an {n/d} star take d turns around a circle, the n vertices of an n-d step prism take d turns around a duocylinder. The n-d step prism is a subsymmetrical faceting of an n-n duoprism. Some step prisms can be swirlchora based on dihedra.

Most often, the cells of step prisms are phyllic disphenoids; in fact, if n and d are coprime, and it does not have doubled symmetry, or if n is odd and has doubled symmetry and is a swirlchoron, it will only contain phyllic disphenoids. However, if n and d are coprime with double symmetry, and is not a swirlchoron, then it will contain tetragonal disphenoids. If n and d are both even, or if n is even and d is coprime with doubled symmetry and the resulting polychoron is a swirlchoron, it will contain rhombic disphenoids, and if n and d have a greatest divisor larger than 2, it will contain antiprism cells, such as the 9-3 step prism with triangular antiprisms and the 15-5 step prism with pentagonal antiprisms.

The duals of the step prisms are called the gyrochora. These are notable, as they make fair dice in four dimensions.

An n-d step prism has double symmetry if n is a divisor of d2+1 or d2-1. Examples include the 13-5 step prism (13 divides 52+1 = 26), 17-4 step prism (17 divides 42+1 = 17), and the 30-11 step prism (30 divides 112-1 = 120).

Step prisms with double symmetry can be compounded to form new isogonal polychora, such as the tetragonal-antiwedge 8-3 double step prism from two 8-3 step prisms.

Construction
To construct the n-d step prism, one starts with a grid of n × n squares. One takes any vertex as the starting vertex, and repeatedly moves one step to the right, and d steps up. One identifies opposite edges of the square sheet: in other words, when one reaches the rightmost or the uppermost edge, one wraps around to the leftmost or to the lowermost edge, respectively. When one reaches the starting vertex, the grid is folded into an n-n duoprism. Finally, one takes the convex hull of all the traversed vertices.

This construction only works for 2 ≤ d ≤ n–2. When d ∈ {0, 1, n–1, n}, all constructed points are coplanar, and the step prism degenerates into a regular polygon.

As a consequence of this construction, the n-d and n-(n–d) step prisms are congruent, as the latter can be constructed from the former by going d steps down instead of d steps up. Furthermore, when n and d are coprime, so that the modular inverse d–1 of d modulo n exists, the n-d–1 and n-(n–d–1) will also be congruent to the aforementioned step prisms, as these can be constructed by exchanging horizontal steps with vertical steps.

Vertex coordinates
Coordinates for the vertices of an n-d step prism with circumradius $\sqrt{2}$ are given by: for k ranging from 0 to n–1.
 * (cos(2πk/n), sin(2πk/n), cos(2πdk/n), sin(2πdk/n)),