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In geometry, the pentagonal antiprism is the third in an infinite set of antiprisms formed by an even-numbered sequence of triangle sides closed by two polygon caps. It consists of two pentagons joined to each other by a ring of 10 triangles for a total of 12 faces. Hence, it is a non-regular dodecahedron.
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Geometry
If the faces of the pentagonal antiprism are all regular, it is a semiregular polyhedron. It can also be considered as a parabidiminished icosahedron, a shape formed by removing two pentagonal pyramids from a regular icosahedron leaving two nonadjacent pentagonal faces; a related shape, the metabidiminished icosahedron (one of the Johnson solids), is likewise form from the icosahedron by removing two pyramids, but its pentagonal faces are adjacent to each other. The two pentagonal faces of either shape can be augmented with pyramids to form the icosahedron.
Relation to polytopes
The pentagonal antiprism occurs as a constituent element in some higher-dimensional polytopes. Two rings of 10 pentagonal antiprisms each bound the hypersurface of the 4-dimensional grand antiprism. If these antiprisms are augmented with pentagonal prism pyramids and linked with rings of 5 tetrahedra each, the 600-cell is obtained.