In the diagram, the two horizontal lines are parallel to each other, and are crossed by a sloping straight line, causing angles to be formed.

The angles marked 'c' are equal to each other, and they are called corresponding angles.

Each 'c' angle has an adjacent angle marked 'a'. As you have already learnt, the sum of each pair of 'a' and 'c' angles will always be 180° (as angles on a straight line).

Also the two 'a' angles are equal to each other, as alternate angles between the two parallel lines.

These equalities can be used the other way too. If you know there are alternate or corresponding angles equal to each other, then you have proved that the lines concerned are parallel. Remember these rules about parallel lines and angles, and the terms corresponding and alternate.