Parts and Properties of an Angle

An angle is formed when two rays meet at a common point called the vertex. 

The measure of an angle is the amount of circular rotation about a point starting with a ray and ending with a second ray.  For this angle, the measure is 80°.

The area within the two rays is called the interior of an angle, represented by the green shading in the picture.  The area outside the two rays is called the exterior of an angle, represented by the yellow shading in the picture.

Benchmark Angles

Here are some angles to help measure angles.  Use these angles to estimate the size of the angle to be drawn. 

All of these angles are 45°.  The manner in which they are turned does not affect the size of the angle.  The rays form a 45° at the vertex.

All of these angles are 120°.  The rays form a 120° at the vertex.

All of these angles are 90°.  The rays form a 90° angle at the vertex.

Angles are Degrees of Rotation

An angle is formed when two rays meet at a common point called the vertex. 

The measure of an angle is the amount of circular rotation about a point starting with a ray and ending with a second ray.  All of these angles measure 45°.