Complementary angle
Two angles that add up to 90 degrees are called complementary angles. If, for example, you have a right angle and you draw a third straight line between the first two lines and if that third line also goes through the vertex of the first two lines you have then split up the first angle into two adjacent angles. If the first angle was 90 degrees then the two new angles must add up to 90 degrees. They are then complementary angles. Each one of these two angles is called a complementary angle. In German the word Winkel is the same in its plural and its singular form. The German word Komplementärwinkel can therefore either mean each one of the two angles or the two angles meant as a pair.