- Birotunda
In
geometry , a birotunda is the name for several differentpolyhedra , formed from two rotundas adjoined through their common regular-decagon faces. In an "orthobirotunda", one of the two rotundas is placed as the mirror reflection of the other, while in a "gyrobirotunda" one rotunda is twisted relative to the other. In addition, a rotunda may be elongated in various ways by a ring of faces that separates one rotunda from the other.There are five birotundas among the
Johnson solid s:* the
pentagonal orthobirotunda ,
* the pentagonal gyrobirotunda, more commonly called theicosidodecahedron ,
* theelongated pentagonal orthobirotunda , in which a ring of ten square faces separates two mirrored rotundas,
* theelongated pentagonal gyrobirotunda , and
* thegyroelongated pentagonal birotunda , in which a ring of twenty triangular faces separates the two rotundas.Another Johnson solid, the
bilunabirotunda , is not a birotunda despite having that word as part of its name.
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