An Octadecagon is an 18-sided polygon with 18 angles.
Petrie polygons[]
The regular octadecagon is the Petrie polygon for a number of higher-dimensional polytopes, shown in these skew orthogonal projections from Coxeter planes:
| A17 |  17-simplex | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| BC9 |  9-orthoplex |
 Rectified 9-orthoplex |
 Birectified 9-orthoplex |
 Trirectified 9-orthoplex |
 Quadrirectified 9-cube |
 Trirectified 9-cube |
 Birectified 9-cube |
 Rectified 9-cube |
 9-cube |
| D10 |  t8(171) |
 t7(171) |
 t6(171) |
 t5(171) |
 t4(171) |
 t3(171) |
 t2(171) |
 t1(171) |
 10-demicube (171) |
| E7 |  (321) |
 t1(321) |
 t2(321) |
 (231) |
 t1(231) |
 t1(132) |
 (132) |
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