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.