Supplementary Angles
From Latin: supplementum - "something added to supply a deficiency"
Definition: Two angles that add up to 180°
Try this Drag an orange dot. The two angles will change so that they always add to 180°

In the figure above, the two angles PQR and JKL are supplementary because they always add to 180°

Often the two angles are adjacent, in which case they form a linear pair like this:

linear pair of angles - two angles that are adjacent (share a leg) and supplementary (add up to 180 deg)

Similar in concept are complementary angles, which add up to 90°.

A way to remember

Sometimes it's hard to remember which is which between supplementary (adds to 180°) and complementary (adds to 90°). Here are two memory aids:

  • The phrase "it's right to give compliments" A right angle is 90 degrees and, yes, 'compliment' and 'complement' are not the same word, but it's a way to remember which is which.
  • Put a vertical line on the right of the letter 'c' in 'complementary' to make into a '9'. Complementary angles add to 90.

Related angle topics

General

Angle Types

Angle relationships