An octagon is a flat shape with eight straight sides and eight corners (also called vertices). The word comes from Greek roots meaning “eight angles.”

The most famous octagon you see every day is probably a stop sign. Traffic engineers chose the octagon for stop signs because its unique eight-sided shape makes it instantly recognizable, even from behind or when covered in snow. No other road sign uses this shape, so drivers know what it means before they can even read the word “STOP.”

Octagons can be regular or irregular. A regular octagon has all eight sides the same length and all eight angles equal, like a stop sign. An irregular octagon still has eight sides, but they can be different lengths with different angles, like a lopsided eight-sided cookie.

You'll find octagons in architecture too. The Dome of the Rock in Jerusalem and many church towers use octagonal designs. Some old houses feature octagonal rooms, and umbrellas often form octagons when you look at them from above. Architects appreciate octagons because they're more interesting than squares but easier to build than circles.