supplementary angles

/ˌsʌpləˈmɛntəri ˈæŋɡəlz/

In geometry, two angles that together add up to 180 degrees are supplementary angles. If you put supplementary angles side by side, they make a flat line.

Supplementary angles get their name from the fact that they supplement, or add something to, each other. The Latin root, supplere, means "complete or supply what is needed." In the case of supplementary angles, it's all about making up a straight, 180-degree line. So you can describe a 120-degree angle and a 60-degree angle as supplementary.

Definitions of supplementary angles

1. noun

   two angles whose sum is 180 degrees
   type of:

   angle

   the space between two lines or planes that intersect; the inclination of one line to another; measured in degrees or radians