In geometry, a **6-demicube** or **demihexteract** is a uniform 6-polytope, constructed from a *6-cube* (hexeract) with alternated vertices truncated. It is part of a dimensionally infinite family of uniform polytopes called demihypercubes.

E. L. Elte identified it in 1912 as a semiregular polytope, labeling it as HM_{6} for a 6-dimensional *half measure* polytope.

Coxeter named this polytope as **1**_{31} from its Coxeter diagram, with a ring on one of the 1-length branches, . It can named similarly by a 3-dimensional exponential Schläfli symbol
{
3
3
,
3
,
3
3
}
or {3,3^{3,1}}.

Cartesian coordinates for the vertices of a demihexeract centered at the origin are alternate halves of the hexeract:

(±1,±1,±1,±1,±1,±1)

with an odd number of plus signs.

There are 47 uniform polytopes with D_{6} symmetry, 31 are shared by the B_{6} symmetry, and 16 are unique:

The 6-demicube, 1_{31} is third in a dimensional series of uniform polytopes, expressed by Coxeter as k_{31} series. The fifth figure is a Euclidean honeycomb, 3_{31}, and the final is a noncompact hyperbolic honeycomb, 4_{31}. Each progressive uniform polytope is constructed from the previous as its vertex figure.

It is also the second in a dimensional series of uniform polytopes and honeycombs, expressed by Coxeter as 1_{3k} series. The next figure is the Euclidean honeycomb 1_{33} and the final is a noncompact hyperbolic honeycomb, 1_{34}.