Had enough of angles?You haven’t even begun! You named angles
and determined their congruence or incongruence when two lines cross. In
this chapter, you will actually measure angles using an instrument called the
protractor, a special ruler used to measure the degrees of angles.
How to Measure an Angle Using a Protractor
Most protractors have two scales along their arc. The lower scale, starting with
zero on the right, is used to measure anglesthat open in a counter-clockwise
arc. To measure a counter-clockwise angle, place the vertex in the circular open-
ing at the base of the protractor and extend the bottom ray through the 0°
mark out to the right. In Figure 4.1, the counter clockwise angle drawn is 20°:
ZYW X(^21)
Figure 4.1