**A polygon that has all sides congruent is called a "regular polygon."** Regular polygons must also have all angles congruent, which is a side effect of having congruent sides. For instance, a square must always have four 90-degree angles because of its four congruent sides.

A regular polygon may have any number of sides, as long as its sides and angles are all congruent. Common examples include five-sided pentagons, six-sided hexagons, seven-sided heptagons, eight-sided octagons and nine-sided nonagons. It is possible for any of these polygons to be irregular if they have the same number of sides but in different sizes.