A right triangle is a triangle in which one angle is a right angle. The side opposite the right angle is called the hypotenuse (side [BC] in the figure below). In addition, the sides adjacent to the right angle are called legs or catheti (singular: cathetus). Side [AB] is called the side adjacent to angle α and opposed to angle β, while side [AC] is the side adjacent to angle β and opposed to angle α. Compared with angle α, side [AB] is said to be the adjacent cathetus and [AC] is the opposite cathetus. Image:Triangle rectangle vect.svg