# Petrie polygon

In geometry, a **Petrie polygon** for a regular polytope of *n* dimensions is a skew polygon in which every (*n* – 1) consecutive sides (but no *n*) belongs to one of the facets. The **Petrie polygon** of a regular polygon is the regular polygon itself; that of a regular polyhedron is a skew polygon such that every two consecutive side (but no three) belongs to one of the faces.^{[1]} Petrie polygons are named for mathematician John Flinders Petrie.

For every regular polytope there exists an orthogonal projection onto a plane such that one Petrie polygon becomes a regular polygon with the remainder of the projection interior to it. The plane in question is the Coxeter plane of the symmetry group of the polygon, and the number of sides, *h,* is Coxeter number of the Coxeter group. These polygons and projected graphs are useful in visualizing symmetric structure of the higher-dimensional regular polytopes.

Petrie polygons can be defined more generally for any embedded graph. They form the faces of another embedding of the same graph, usually on a different surface, called the Petrie dual.^{[2]}

John Flinders Petrie (1907–1972) was the only son of Egyptologist Flinders Petrie. He was born in 1907 and as a schoolboy showed remarkable promise of mathematical ability. In periods of intense concentration he could answer questions about complicated four-dimensional objects by *visualizing* them.

He first noted the importance of the regular skew polygons which appear on the surface of regular polyhedra and higher polytopes. Coxeter explained in 1937 how he and Petrie began to expand the classical subject of regular polyhedra:

In 1938 Petrie collaborated with Coxeter, Patrick du Val, and H.T. Flather to produce *The Fifty-Nine Icosahedra* for publication.^{[4]}
Realizing the geometric facility of the skew polygons used by Petrie, Coxeter named them after his friend when he wrote *Regular Polytopes*.

The idea of Petrie polygons was later extended to semiregular polytopes.

Infinite regular skew polygons (apeirogon) can also be defined as being the Petrie polygons of the regular tilings, having angles of 90, 120, and 60 degrees of their square, hexagon and triangular faces respectively.

The Petrie polygon projections are useful for the visualization of polytopes of dimension four and higher.

A hypercube of dimension *n* has a Petrie polygon of size 2*n*, which is also the number of its facets.

So each of the (*n*−1)-cubes forming its surface has *n*−1 sides of the Petrie polygon among its edges.

This table represents Petrie polygon projections of 3 regular families (simplex, hypercube, orthoplex), and the exceptional Lie group E_{n} which generate semiregular and uniform polytopes for dimensions 4 to 8.