In general two things are complementary when they add up to a whole of some kind. For example, in geometry, two angles are said to be complementary when they add up to 90°. One angle is said to be the complement of the other. In the figure below, angles PQR and RQS are complementary.
RQS is the complement of PQR. See Complementary angles for more.
Definition and properties of complementary angles - two angles that add to 90 degrees
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