Supplementary angle
Two angles are called supplementary if they add up to 180 degrees. Their geometrical role is not necessarily important. They may be adjacent angles on a straight line or any other two angles. One of the two angles is called a supplementary angle. Note that in German the word Winkel (angle) looks the same in its plural and its singular form. This is why both "supplementary angles" as well as "supplementary angle" can both be translated into the German word Supplementärwinkel.