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Wiktionary
birotunda

n. A structure made of two adjoined rotundas.

Wikipedia
Birotunda

bgcolor=#e7dcc3 colspan=2|Set of birotundas

align=center colspan=2|
(Example Ortho/gyro pentagonal forms)

bgcolor=#e7dcc3|Faces

bgcolor=#e7dcc3|Edges

bgcolor=#e7dcc3|Vertices

bgcolor=#e7dcc3| Symmetry group

bgcolor=#e7dcc3| Rotation group

bgcolor=#e7dcc3|Properties

In geometry, a birotunda is any member of a family of dihedral-symmetric polyhedra, formed from two rotunda adjoined through the largest face. They are similar to a bicupola but instead of alternating squares and triangles, it alternates pentagons and triangles around an axis. There are two forms, ortho- and gyro-: an orthobirotunda has 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.

The pentagonal birotundas can be formed with regular faces, one a Johnson solid, the other a semiregular polyhedron:

  • pentagonal orthobirotunda,
  • pentagonal gyrobirotunda, which is also called an icosidodecahedron.

Other forms can be generated with dihedral symmetry and distorted equilateral pentagons.