xiii. When the sum of two angles BAC, CAD is such that the legs BA, AD form one right line, they are called supplements of each other.
Hence, when one line stands on another, the two angles which it makes on the same side of that on which it stands are supplements of each other.
xiv. When one line stands on another, and makes the adjacent angles at both sides of itself equal, each of the angles is called a right angle, and the line which stands on the other is called a perpendicular to it.
Hence a right angle is equal to its supplement.
xv. An acute angle is one which is less than a right angle, as A.
xvi. An obtuse angle is one which is greater than a right angle, as BAC.
The supplement of an acute angle is obtuse, and conversely, the supplement of an obtuse angle is acute.
