FORMULABANK

When a line (shown in red) intersects two other lines (shown in black), four pairs of corresponding angles will be formed. In the example on the right, the corresponding angles are marked with the same color.

Two angles are corresponding when the red intersecting line forms:
• either the left ray in both angles, and the right ray is formed by each of the black lines.
• or the right ray in both angles, and the left ray is formed by each of the black lines.
(It is often additionally said that the two angles should be on the same side of the red intersecting line. This is the case in our example, but it can be debated.)

If just one pair of corresponding angles has the same size, then the two black lines will be parallel.

If the two black lines are parallel, then the corresponding angles will be of the same size.