Polygon, << POL ee gon, >> is a closed plane figure composed of a certain number of straight line segments called sides. For example, a polygon that has three sides is a triangle. A polygon with five sides is a pentagon, and one with eight sides is an octagon. But there are no special names for polygons with a large number of sides, such as one with 128 sides or one with 3,472 sides.
The end points of the sides of a polygon are called vertexes or vertices. Two sides—and only two sides—meet at each vertex. Those neighboring sides are known as adjacent sides. Where adjacent sides meet, they form an angle inside the polygon. Two sides that are not adjacent do not meet at any point.
A polygon is called equilateral if all its sides are equal in length. It is equiangular if all its angles are equal. A polygon that is both equilateral and equiangular is known as a regular polygon. A regular polygon is a type of convex polygon. A polygon is convex if no side, when extended, enters the polygon. A polygon is concave if any side, when extended, enters the polygon.
The diagram that accompanies this article may help you understand the following three facts about regular polygons: (1) You can draw a circle around a regular polygon so that all of the polygon’s vertexes lie on the circle. (2) The center of that circle defines the center of the polygon. (3) Drawing a straight line segment—called a radius—from the center of the polygon to each vertex will divide the polygon into identical triangles, one for each side of the polygon.
