
A polygon is a 2dimensional closed figure that is the union of line segments in a plane. A polygon has three or more sides. 

You already have a good understanding of 3sided and 4sided polygons from your previous studies of geometry. We will refresh and enhance those previous concepts as we add some new information.
Classifications of Polygons 
• Polygons can be classified as either regular or irregular.
In a regular polygon, all of its interior angles are of the same measure and all of its sides are the same length.

In an irregular polygon, the interior angles may be of different measures and the sides may be different lengths.

A polygon that is regular is both equilateral AND equiangular.
A regular triangle is an equilateral triangle (and an equiangular triangle).
A regular quadrilateral is a square. 

The term "polygon" refers to a type of figure which may be regular or irregular. Do NOT assume "polygon" means "regular polygon". If a polygon is a regular polygon, it will be stated as such. 
• Polygons can be classified as either convex or concave.
A polygon is convex if no line that contains a side of the polygon contains a point in the interior of the polygon. In a convex polygon, each interior angle measures less than 180 degrees. A triangle is always convex, and regular polygons are always convex.
Concave polygons "cavein" to their interiors, creating at least one interior angle greater than 180 degrees (a reflex angle).


Parts of Regular Polygons 
Regular Pentagon

The center, (O), of a regular polygon, is a point in the interior of the polygon equidistant from each vertex. It is also the center of the inscribed and circumscribed circles. The radius is the distance from the center to any vertex.
The central angle of a regular polygon is formed by connecting the center to two adjacent vertices. All central angles are of equal measure in a regular polygon. 
Irregular polygons are not described as having a "center" which is equidistant from each vertex. As such, an irregular polygon does not have central angles or radii. An irregular polygon does, however, have a center of gravity or a centroid. 
The table below shows some of the more commonly used polygons.
Do NOT assume that the diagrams in the "Figure" column are "regular" polygons.
Polygon 
Figure 
Sides 
Angles 
Diagonals 
Number
Triangles 
Triangle 

3 
3 
0 
1 
Quadrilateral 

4 
4 
2 
2 
Pentagon 

5 
5 
5 
3 
Hexagon 

6 
6 
9 
4 
Heptagon or Septagon 

7 
7 
14 
5 
Octagon 

8 
8 
20 
6 
Nonagon or Novagon 

9 
9 
27 
7 
Decagon 

10 
10 
35 
8 
Dodecagon 

12 
12 
54 
10 
ngon 
 
n 
n 
½n(n  3) 
(n  2) 
An ngon is a polygon with n sides.