In geometry, a **9-orthoplex** or 9-cross polytope, is a regular 9-polytope with 18 vertices, 144 edges, 672 triangle faces, 2016 tetrahedron cells, 4032 5-cells *4-faces*, 5376 5-simplex *5-faces*, 4608 6-simplex *6-faces*, 2304 7-simplex *7-faces*, and 512 8-simplex *8-faces*.

It has two constructed forms, the first being regular with Schläfli symbol {3^{7},4}, and the second with alternately labeled (checkerboarded) facets, with Schläfli symbol {3^{6},3^{1,1}} or Coxeter symbol **6**_{11}.

It is one of an infinite family of polytopes, called cross-polytopes or *orthoplexes*. The dual polytope is the 9-hypercube or enneract.

**Enneacross**, derived from combining the family name *cross polytope* with *ennea* for nine (dimensions) in Greek
**Pentacosidodecayotton** as a 512-facetted 9-polytope (polyyotton)
There are two Coxeter groups associated with the 9-orthoplex, one regular, dual of the enneract with the C_{9} or [4,3^{7}] symmetry group, and a lower symmetry with two copies of 8-simplex facets, alternating, with the D_{9} or [3^{6,1,1}] symmetry group.

Cartesian coordinates for the vertices of a 9-orthoplex, centered at the origin, are

(±1,0,0,0,0,0,0,0,0), (0,±1,0,0,0,0,0,0,0), (0,0,±1,0,0,0,0,0,0), (0,0,0,±1,0,0,0,0,0), (0,0,0,0,±1,0,0,0,0), (0,0,0,0,0,±1,0,0,0), (0,0,0,0,0,0,±1,0,0), (0,0,0,0,0,0,0,±1,0), (0,0,0,0,0,0,0,0,±1)

Every vertex pair is connected by an edge, except opposites.