What Are Supplementary Angles?

Supplementary angles add up to $180^\circ$

When two angles' measures add up to $180^\circ$, the angles are called supplementary (to each other). Supplementary pairs can either be made up of two right angles, or one acute and one obtuse angle.

In the image above, angles ABC and DEF are supplementary. We can tell because they're congruent with angles GHJ and JHI (respectively), and we can tell from the picture that GHJ and JHI add up to make GHI... which is just a straight line (and thus technically has a measure of $180^\circ$.

Angles GHJ and JHI also form a linear pair.