An icosagon is a 20-sided polygon with 20 angles.
Petrie polygons
The regular icosagon is the Petrie polygon for a number of higher-dimensional polytopes, shown in orthogonal projections in Coxeter planes:
| A9 |  19-simplex (19D) | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| BC10 |  10-orthoplex |
 Rectified 10-orthoplex |
 Birectified 10-orthoplex |
 Trirectified 10-orthoplex |
 Quadrirectified 10-orthoplex |
 Quadrirectified 10-cube |
 Trirectified 10-cube |
 Birectified 10-cube |
 Rectified 10-cube |
| D11 |  11-demicube | ||||||||
| E8 [20] |
 (421) |
 t1(421) |
 t2(421) |
 t3(421) |
 t4(421) |
 t0(241) |
 t1(241) |
 t0(142) | |
| H4 [20] |
 Rectified 600-cell |
 600-cell |
| ||||||||||||||||||||
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