Cross-section

A cross-section is informally the part of a polytope that is exposed after being cut with a hyperplane.

More formally, it is the intersection of the sets of points "inside" a polytope and inside a hyperplane. However, for polytopes without well-defined interiors, the set of points inside the polytope's facets can also work.

Taking the cross-section of a polytope at its vertex, in an appropriate direction, should result in the polytope's vertex figure.

For viewing higher-dimensional polytopes
The structures of higher-dimensional polytopes can be difficult to understand. Viewing cross-sections of these polytopes can provide insights into how they're laid out. Projections are another way to do this.

For example, this is a series of cross-sections of the icositetrachoron. The cross-sections span (from left to right, then to the start of the next row down) from an octahedral cell up to the widest point on the icositetrachoron (halfway through it, and equivalent to a cuboctahedron). All are taken in parallel hyperplanes to the starting octahedral cell.

The cells we can see are:


 * An octahedral cell at the start
 * Six halves of octahedral cells coming off of the first cell's vertices. These are visible as growing squares.
 * In this direction, squares are the cross-sections of the octahedron.
 * Eight octahedral cells coming off of the first cell's faces. These are visible as triangles "morphing" into dual triangles.
 * In this direction, triangles and (isogonal) hexagons are the cross-sections of the octahedron.