If two straight lines cross each other, four angles are formed. The sum of all these angles (a, b, c, d) will always be 360°.

If any number of straight lines meet at the same point, the sum of the angles formed by them will always be 360°.

The angles vertically opposite each other when straight lines cross will always be equal. In the diagram 'a' equals 'c' and 'b' equals 'd'.

If any number of straight lines meet at the same point of another straight line, the sum of the angles formed on a straight line will always be 180°. Here the angles a, b, and c, add up to 180°.

Look again at the top diagram. This straight line rule means that a + b = 180°, and c +d = 180°.