ALTERNATE Angles, in Geometry, are the internal Angles made by a Line cutting two Parallels, and lying on the opposite Sides of the cutting Line; the one below the first Parallel, and the other above the second. See

ANGLE. Thus, a and b, and x and y, (Tab. Geometry, Fig. 46.) are Alternate Angles. There are also two external Angles, alternately opposite to the internal ones. See

OPPOSITE. Alligation