Supplementary Angles


2 Angles are Supplementary when they add up to 180 degrees.

Supplement comes from Latin supplere, to complete or “supply” what is needed.

Supplementary Angles

These two angles (120° and 60°) are Supplementary Angles, because they add up to 180° and make a straight angle.

But the angles don’t have to be together.

Supplementary Angles Each Other

These 2 angles are supplementary because 120° + 60° = 180°

When the two angles add to 180°, we say they “Supplement” each other.

Supplementary Vs Complementary

A related idea is Complementary Angles, they add up to 90°

How to remember which is which? Just think:

“S” of Supplementarystands for “Straight” (a straight line)
“C” of Complementarystands for “Corner” (a Right Angle)

Learn More

Complementary Angles

Congruent Angles

How To Find if Triangles are Congruent

Adjacent Angles

Geometry Index