In order to be able to identify the icosahedron 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:
Vertex-first parallel projection.
Edge-first parallel projection. Four of the icosahedron's faces project onto the top two and bottom two edges of the projection envelope.
Face-first parallel projection.
Parallel projection, parallel to two opposite faces. The two opposite faces project to the top and bottom edges of the projection envelope.
The Cartesian coordinates of an origin-centered regular icosahedron of edge length 2 are:
- (0, ±1, ±φ)
- (±1, ±φ, 0)
- (±φ, 0, ±1)
where φ=(1+√5)/2 is the Golden Ratio.
The icosahedron occurs as cells in the following 4D uniform polytopes:
In addition, it also occurs in many CRF polychora, including (but not limited to):