2.3 Positive and Negative Angle Measures
Angle measures must be either positive, negative, or zero. This is because
if an angle has a measure of x when measured clockwise, then it must
have a measure of -x when measured counter-clockwise.
It is customary in mathematics that measuring an angle counter-clockwise
produces positive measures and measuring clockwise produces negative
measures. An easy way to remember positive and negative directions for
measuring angles is to place your right fist on a flat surface. Your fingers
curl in the positive direction when your thumb points up.