12 Preliminaries Ch. 1
A
B
C
Figure 1.6. An interior region.A
B
C
The corresponding exterior region.- If|BACis a non-straight angle-support, then the couples
(|BAC,IR(|BAC)),(|BAC,ER(|BAC)),are called thewedge-angleandreflex-
angle, respectively, withsupport|BAC; this wedge-angle is denoted by∠BAC. Thus
a wedge-angle is a pair of arms in association with an interior region, while a reflex-
angle is a pair of arms combined with an exterior region.
If|BACis a straight angle-support, andH 1 ,H 2 are the closed half-planes with
edge the lineAB, then the couples(|BAC,H 1 ),(|BAC,H 2 ),are called thestraight-
angleswithsupport|BAC.IfC∈[A,Bthen the wedge-angle∠BAC=∠BABis called
anull-angle, and the reflex-angle with support|BABis called afull-angle.
Figure 1.7. A wedge-angle. A reflex-angle.A straight-angle.NOTE. The reason we call|BACan angle-support and not an angle is that it sup-
ports two angles. If we were confining ourselves to pure geometry, and not concerned
to go forward to coordinate geometry and trigonometry, we could confine ourselves
to wedge and straight angles. Even more if we were to confine ourselves to the angles