Tetrahemihexahedron

The tetrahemihexahedron, tetrahemicube, or thah, is a quasiregular polyhedron and one of 10 uniform hemipolyhedra. It consists of 4 equilateral triangles and 3 "hemi" squares passing through the center, with two of each joining at a vertex. It can be derived as a facetorectified tetrahedron. Its triangular faces are parallel to those of a tetrahedron, and its hemi square faces are parallel to those of a cube or hexahedron: hence the name. It is also the 3D demicross. It can be derived as a rectified petrial tetrahedron.

It is the only nonconvex uniform polyhedron that only has A3 symmetry in its highest symmetry form. It's also the only uniform polyhedron, other than prisms whose bases have an odd amount of sides, with an odd amount of faces. It is also the only uniform polyhedron with an odd Euler characteristic.

The visible portion of this solid resembles an octahedron with four triangular pyramids carved out. In fact the four triangular faces are a tetrahedral subset of those of an octahedron, while the squares are the 3 equatorial planes of the octahedron. It also shares its vertices and edges with the octahedron.

It also happens to be a 3/2-gonal cuploid (retrograde triangular cuploid), as it can be formed from a retrograde triangular cupola by removing the bottom degenerate face.

Tiling of the projective plane
The tetrahemihexahedron can be thought of as a tiling of the real projective plane. This makes it non-orientable and gives it Euler characteristic 1.

The double-cover of the tetrahemihexahedron is isomorphic to the cuboctahedron, and thus it is a quotient of the cuboctahedron, where antipodal elements are identified. Conversely, quasicantellating a tetrahedron produces a doubly-covered tetrahemihexahedron.

Vertex coordinates
Its vertices are the same as those of its regiment colonel, the octahedron.

In vertex figures
The tetrahemihexahedron appears as a vertex figure in one uniform polychoron, that being the tesseractihemioctachoron. It has an edge length of 1.

Irregular tetrahemihexahedra appear as vertex figures of several uniform polychora in Bowers' category 11, namely piphid, stefacoth, shafipto, shif phix, six fipady, hi fipady, mohi fipady, six fixady, shi fixady, gohi fohixhi, and gaxifthi.

Polyhedra isomorphic to the tetrahemihexahedron appear as vertex figures of several uniform polychora in Bowers' category 12, namely sto, gittifcoth, gahfipto, gix fixady, gohi fixady, ghif phix, gix fipady, shi fohixhi, and saxifthi.

Related polyhedra
Two uniform polyhedron compounds are composed of tetrahemihexahedra, both of which share edges with compounds of octahedra:


 * Hemirhombichiricosahedron (5)
 * Snub pseudosnub rhombicosahedron (20)