Close approximations to the regular hendecagon can be constructed, however. An 11-sided polygon is called a hendecagon or undecagon. The cross-section of a loonie is actually a Reuleaux hendecagon. In geometry, a hendecagon (also undecagon or endecagon) or 11-gon is an eleven-sided polygon. A regular hendecagon has internal angles of 147.27 degrees (=147 formula 1 degrees).