The Rhombicuboctahedron
The rhombicuboctahedron, also known as the small rhombicuboctahedron, is a 3D uniform polyhedron bounded by 8 triangles and 6+12=18 squares. It may be constructed by radially expanding the square faces of the cube outwards, or equivalently, radially expanding the triangular faces of the octahedron outwards.
There are two distinct kinds of square faces on the rhombicuboctahedron: the first kind are the axial faces which are surrounded by 4 other square faces. There are six of these faces, and they correspond with the faces of a cube:
The second kind of square face are the nonaxial faces, which are surrounded by 2 squares and 2 triangles. There are 12 of them, and they correspond with the edges of a cube:
It is important to distinguish between these two kinds of square faces, because their relative position to the triangular faces gives them different functions when the rhombicuboctahedron is fitted together with other polyhedra into 4D polytopes.
Projections
In order to be able to identify the rhombicuboctahedron in various projections of 4D objects, it is useful to know how it appears from various viewpoints. The following are some of the commonlyencountered views:
Projection  Envelope  Description 

Regular octagon  Parallel projection centered on an axial square face. 

Regular hexagon  Trianglecentered parallel projection. 

Octagon  Parallel projection centered on nonaxial square face. 

Octagon  Vertexcentered parallel projection. 
Geometric Relations
The rhombicuboctahedron can be cut into two square cupolae (J4) and an octagonal prism, or into a square cupola and an elongated square cupola (J19). It can also be gyrated to form the gyroelongated square bicupola (J37). The latter has the same configuration of faces around every vertex (3 squares and a triangle), but is not vertextransitive, hence it is not uniform, but is a Johnson solid.
Coordinates
The Cartesian coordinates of the rhombicuboctahedron, centered on the origin and having edge length 2, are all permutations of coordinates and changes of sign of:
 (1, 1, (1+√2))
Occurrences
The rhombicuboctahedron appears as cells in the following 4D uniform polytopes:
 The cantellated tesseract, its direct 4D analogue;
 The runcitruncated 16cell;
 The cantellated 24cell;
 The runcitruncated 24cell.