Parallel Lines

Parallel lines are always the same distance apart (“equidistant”) and will never meet. Examiners love to throw in a question or two about them.

Angles Around Parallel Linesangles around parallel linesThe arrows in the above diagram show the lines are parallel.

When a line crosses 2 parallel line only 2 different angles are actually formed and these that add up to 180°.

The vertically opposite angles are equal. but as you can see in the diagram above there is a lot more going on.

Alternate Anglesalternate anglesAlternate angles are equal. You can see them in the Z shape.

Supplementary Anglessupplementary anglesSupplementary angles add up to 180°. You can see them in the C shape (sometimes it will look more like a U if the parallel lines were going vertically rather than horizontally).

Here a + b = 180°

Corresponding Anglescorresponding anglesCorresponding ages are equal. You can find them in the F shape.


These are quadrilaterals made up of two sets of parallel lines. angles in a parallelogramYou can see from the rules above that angles B and D are equal. Also angles A and C are equal.

Also A and D add up to 180°. So do angles B and C.