Plane figures are flat. They can be closed or not closed.

Plane figures made up of three or more closed line segments are polygons. Each line segment of a polygon is a side. Polygons are classified by the number of sides.

A triangle has three sides. There are different kinds of triangles.

- An equilateral triangle has three sides of equal length.
- An isosceles triangle has only two sides of equal length.
- A right triangle has one right angle.