<Dd> (+ 1, − 1, − 1). </Dd> <P> This yields a tetrahedron with edge - length 2 √ 2, centered at the origin . For the other tetrahedron (which is dual to the first), reverse all the signs . These two tetrahedra's vertices combined are the vertices of a cube, demonstrating that the regular tetrahedron is the 3 - demicube . </P> <P> The volume of this tetrahedron is one - third the volume of the cube . Combining both tetrahedra gives a regular polyhedral compound called the compound of two tetrahedra or stella octangula . </P> <P> The interior of the stella octangula is an octahedron, and correspondingly, a regular octahedron is the result of cutting off, from a regular tetrahedron, four regular tetrahedra of half the linear size (i.e., rectifying the tetrahedron). </P>

What does a net of a sphere look like