Editing 16-cell (section)

=== Structure ===
The [[Schläfli symbol]] of the 16-cell is {3,3,4}, indicating that its cells are [[Regular tetrahedron|regular tetrahedra]] {3,3} and its [[vertex figure]] is a [[regular octahedron]] {3,4}. There are 8 tetrahedra, 12 triangles, and 6 edges meeting at every vertex. Its [[edge figure]] is a square. There are 4 tetrahedra and 4 triangles meeting at every edge.

The 16-cell is [[Totally bounded|bounded]] by 16 [[cell (mathematics)|cells]], all of which are regular [[tetrahedron|tetrahedra]].{{Efn|The boundary surface of a 16-cell is a finite 3-dimensional space consisting of 16 tetrahedra arranged face-to-face (four around one). It is a closed, tightly curved (non-Euclidean) 3-space, within which we can move straight through 4 tetrahedra in any direction and arrive back in the tetrahedron where we started. We can visualize moving around inside this tetrahedral [[jungle gym]], climbing from one tetrahedron into another on its 24 struts (its edges), and never being able to get out (or see out) of the 16 tetrahedra no matter what direction we go (or look). We are always on (or in) the ''surface'' of the 16-cell, never inside the 16-cell itself (nor outside it). We can see that the 6 edges around each vertex radiate symmetrically in 3 dimensions and form an orthogonal 3-axis cross, just as the radii of an octahedron do (so we say the vertex figure of the 16-cell is the octahedron).{{Efn|name=octahedral pyramid}}}} It has 32 [[triangle (geometry)|triangular]] [[face (geometry)|faces]], 24 [[edge (geometry)|edges]], and 8 [[vertex (geometry)|vertices]]. The 24 edges bound 6 [[orthogonal]] central squares lying on [[great circles]] in the 6 coordinate planes (3 pairs of completely orthogonal great squares). At each vertex, 3 great squares cross perpendicularly. The 6 edges meet at the vertex the way 6 edges meet at the [[Apex (geometry)|apex]] of a canonical [[octahedral pyramid]].{{Efn|Each vertex in the 16-cell is the apex of an [[octahedral pyramid]], the base of which is the octahedron formed by the 6 other vertices to which the apex is connected by edges. The 16-cell can be deconstructed (four different ways) into two octahedral pyramids by cutting it in half through one of its four octahedral central hyperplanes. Looked at from inside the curved 3 dimensional volume of its boundary surface of 16 face-bonded tetrahedra, the 16-cell's vertex figure is an octahedron. In 4 dimensions, the vertex octahedron is actually an octahedral pyramid. The apex of the octahedral pyramid (the vertex where the 6 edges meet) is not actually at the center of the octahedron: it is displaced radially outwards in the fourth dimension, out of the hyperplane defined by the octahedron's 6 vertices. The 6 edges around the vertex make an orthogonal 3-axis cross in 3 dimensions (and in the [[Octahedral pyramid|3-dimensional projection of the 4-pyramid]]), but the 3 lines are actually bent 90 degrees in the fourth dimension where they meet in an apex.|name=octahedral pyramid}} The 6 orthogonal central planes of the 16-cell can be divided into 4 orthogonal central hyperplanes (3-spaces) each forming an [[octahedron]] with 3 orthogonal great squares.