Angles whose measures add up to 90°.
Examples
In this figure, angles
A and
B are complementary, since the sum of their measures is 90°.
Since these angles are adjacent, they form a right angle.

In the figure below, where a pair of parallel lines intersects another pair of parallel lines, angles
A and
B are also complementary, even though they are not adjacent. In fact, a
translation can be applied to make the vertices of angle
A and angle
B coincide and form a right angle.