The Dodecahedron
The regular dodecahedron is a Platonic solid bounded by 12 regular pentagons. It has 20 vertices and 30 edges.
The dual of the dodecahedron is the icosahedron. The dodecahedron occurs as cells in the 120cell and the runcinated 120cell.
Projections
In order to be able to identify the dodecahedron in various projections of 4D objects, it is useful to know how it appears from various viewpoints. The following are some of the viewpoints that are commonly encountered:
Projection  Envelope  Description 

Nonregular dodecagon  Vertexfirst parallel projection. 

Nonregular hexagon  Edgefirst parallel projection. Four of the dodecahedron's faces project onto 4 of the edges of the hexagonal envelope. 

Regular decagon  Facefirst parallel projection. 

Nonregular octagon  Parallel projection parallel to top and bottom faces. The top and bottom faces project to the top and bottom edges of the projection envelope. 
Coordinates
The canonical Cartesian coordinates for the dodecahedron are:
 (±1, ±1, ±1)
 (0, ±φ^{−1}, ±φ)
 (±φ^{−1}, ±φ, 0)
 (±φ, 0, ±φ^{−1})
where φ=(1+√5)/2 is the Golden Ratio.
These coordinates give a dodecahedron with edge length 2/φ, or approximately 1.236.