Angles are classified, or grouped, according to the size of the opening between
the two rays.   

Angles are measured with a standard unit of measure called degrees.  The symbol for degrees is (°). 

A protractor, like the one on the left, is a tool used to draw and measure the size of angles.  

It shows the number of degrees in a half circle.  It can be used to measure angles from 0° to 180°.

An angle that measures 180° is called a straight angle.  In the picture to the right the red line shows a straight angle.