The sides and arcs are called the edges of the figure. The endpoints of the edges are called the vertices or corners . If all the edges are segments, every vertex is the endpoint of two sides, and no two sides cross each other, the figure is called a polygon . 1 Polygons are classified according to the number of sides they have, which equals the number of vertices. Here are some names of polygons.
Polygons often divide the plane into two pieces, an inside and an outside.
The inside part is called the region enclosed by the figure. The name of the figure is also commonly used for this region, and the area of the region is commonly called the area of the figure. When two sides meet at a vertex, they form an angle. Actually they form two angles, one inside the figure, and one outside. The one inside is called the interior angle at that vertex, or simply the angle at that vertex.