How many sides does a dodecagon have?

A dodecagon has twelve sides. The prefix dodeca comes from the Greek for twelve, combining deka for ten with duo for two, and once you spot it you will recognise it elsewhere, most obviously in the dodecahedron, the twelve-faced Platonic solid. The interior angles of a regular dodecagon are 150 degrees each, and they total 1800 degrees, following the general rule that a polygon with n sides has interior angles summing to 180 times n minus 2. Because 150 does not divide evenly into 360, regular dodecagons cannot tile a plane alone, but they combine happily with triangles and squares in semi-regular tilings. Twelve-sided shapes turn up in coinage, where the distinctive outline helps people and vending machines identify a coin by touch, and in architecture for towers and windows. As the side count of a regular polygon climbs, the shape gets steadily closer to a circle, and a dodecagon already looks nearly round from a distance.

A regular dodecagon has an area of exactly three times the square of its circumradius, an unusually tidy result.