In geometry, a 6-orthoplex, or 6-cross polytope, is a regular 6-polytope with 12 vertices, 60 edges, 160 triangle faces, 240 tetrahedron cells, 192 5-cell 4-faces, and 64 5-faces.
It has two constructed forms, the first being regular with Schläfli symbol {3<sup>4</sup>,4}, and the second with alternately labeled (checkerboarded) facets, with Schläfli symbol {3,3,3,3<sup>1,1</sup>} or Coxeter symbol 3<sub>11</sub>.
It is a part of an infinite family of polytopes, called cross-polytopes or orthoplexes. The dual polytope is the 6-hypercube, or hexeract.
This configuration matrix represents the 6-orthoplex. The rows and columns correspond to vertices, edges, faces, cells, 4-faces and 5-faces. The diagonal numbers say how many of each element occur in the whole 6-orthoplex. The nondiagonal numbers say how many of the column's element occur in or at the row's element.
There are three Coxeter groups associated with the 6-orthoplex, one regular, dual of the hexeract with the C<sub>6</sub> or [4,3,3,3,3] Coxeter group, and a half symmetry with two copies of 5-simplex facets, alternating, with the D<sub>6</sub> or [3<sup>3,1,1</sup>] Coxeter group. A lowest symmetry construction is based on a dual of a 6-orthotope, called a 6-fusil.
Cartesian coordinates for the vertices of a 6-orthoplex, centered at the origin are
Every vertex pair is connected by an edge, except opposites.
The 6-orthoplex can be projected down to 3-dimensions into the vertices of a regular icosahedron.
It is in a dimensional series of uniform polytopes and honeycombs, expressed by Coxeter as 3<sub>k1</sub> series. (A degenerate 4-dimensional case exists as 3-sphere tiling, a tetrahedral hosohedron.)
This polytope is one of 63 uniform 6-polytopes generated from the B<sub>6</sub> Coxeter plane, including also the regular 6-cube.