Pentahedron

In geometry, a pentahedron (pl.: pentahedra) is a polyhedron with five faces or sides. There are no face-transitive polyhedra with five sides, and there are two distinct topological types. Notable polyhedra with regular polygon faces are:

• 
  Square pyramid with four triangles and one square.^[1] Pyramids with any quadrilateral base have the same number of faces.
• 
  Triangular prism with three rectangles and two triangular bases.^[1] In the case of a right triangular prism, it is a special case of wedge with connecting parallel edges between triangles; the wedge generally has two triangles and three quadrilateral faces.^[2] Topologically, the triangular frustum is the same polyhedron, but the two triangles are different sizes, and the sides are slanted trapezoids.

For the Sylvester pentahedron of a cubic surface, see quaternary cubic.

The pentahedra can be used as space-filling.^[3][4]

Concave

Above: a concave pentahedron viewed from its apex. Below: the same object viewed from its concave face.

An irregular pentahedron can be a non-convex solid: Consider a non-convex (planar) quadrilateral (such as a dart) as the base of the solid, and any point not in the base plane as the apex.

Hosohedron

There is a third topological polyhedral figure with 5 faces, degenerate as a polyhedron: it exists as a spherical tiling of digon faces, called a pentagonal hosohedron with Schläfli symbol {2,5}. It has 2 (antipodal point) vertices, 5 edges, and 5 digonal faces.