# Skew polygon

In geometry, a **skew polygon** is a polygon whose vertices are not all coplanar. Skew polygons must have at least four vertices. The *interior* surface (or area) of such a polygon is not uniquely defined.

Skew infinite polygons (apeirogons) have vertices which are not all colinear.

A **zig-zag skew polygon** or **antiprismatic polygon**^{[1]} has vertices which alternate on two parallel planes, and thus must be even-sided.

**Regular skew polygons** in 3 dimensions (and regular skew apeirogons in two dimensions) are always zig-zag.

A **regular skew polygon** is isogonal with equal edge lengths. In 3 dimensions a regular skew polygon is a **zig-zag skew** (or **antiprismatic**) **polygon**, with vertices alternating between two parallel planes. The side edges of an *n*-antiprism can define a regular skew 2*n*-gon.

Examples are shown on the uniform square and pentagon antiprisms. The star antiprisms also generate regular skew polygons with different connection order of the top and bottom polygons. The filled top and bottom polygons are drawn for structural clarity, and are not part of the skew polygons.

A regular compound skew 2*n*-gon can be similarly constructed by adding a second skew polygon by a rotation. These share the same vertices as the prismatic compound of antiprisms.

Petrie polygons are regular skew polygons defined within regular polyhedra and polytopes. For example, the five Platonic solids have 4-, 6-, and 10-sided regular skew polygons, as seen in these orthogonal projections with red edges around their respective projective envelopes. The tetrahedron and the octahedron include all the vertices in their respective zig-zag skew polygons, and can be seen as a digonal antiprism and a triangular antiprism respectively.

A regular skew polyhedron has regular polygon faces, and a regular skew polygon vertex figure.

Three infinite regular skew polyhedra are space-filling in 3-space; others exist in 4-space, some within the uniform 4-polytopes.

An **isogonal skew polygon** is a skew polygon with one type of vertex, connected by two types of edges. Isogonal skew polygons with equal edge lengths can also be considered quasiregular. It is similar to a zig-zag skew polygon, existing on two planes, except allowing one edge to cross to the opposite plane, and the other edge to stay on the same plane.

Isogonal skew polygons can be defined on even-sided n-gonal prisms, alternatingly following an edge of one side polygon, and moving between polygons. For example, on the vertices of a cube. Vertices alternate between top and bottom squares with red edges between sides, and blue edges along each side.

In 4 dimensions, a regular skew polygon can have vertices on a Clifford torus and related by a Clifford displacement. Unlike zig-zag skew polygons, skew polygons on double rotations can include an odd-number of sides.

The Petrie polygons of the regular 4-polytopes define regular skew polygons. The Coxeter number for each coxeter group symmetry expresses how many sides a Petrie polygon has. This is 5 sides for a 5-cell, 8 sides for a tesseract and 16-cell, 12 sides for a 24-cell, and 30 sides for a 120-cell and 600-cell.

When orthogonally projected onto the Coxeter plane, these regular skew polygons appear as regular polygon envelopes in the plane.

The *n*-*n* duoprisms and dual duopyramids also have 2*n*-gonal Petrie polygons. (The tesseract is a 4-4 duoprism, and the 16-cell is a 4-4 duopyramid.)