The word 'alternate' is usually used with pairs of angles, to indicate that each is on opposite sides of a line.
In the figure below, the two angles are called alternate angles because they are on opposite sides of the sloping
Definition and properties of alternate exterior angles, showing the pairs of angles associated with a transversal
Definition and properties of alternate interior angles, showing the pairs of angles associated with a transversal
(C) 2009 Copyright Math Open Reference. All rights reserved