alternate angles 
plural noun Mathematics
Two angles, formed when a line crosses two other lines, that lie on opposite sides of the transversal line and on opposite relative sides of the other lines. If the two lines crossed are parallel, the alternate angles are equal.The cross-ply tyre is made up of superimposed layers of textile cord running at alternate angles from bead to bead....
- So in the figure above, as you move points A or B, the two alternate angles shown always have the same measure.