They have a right angle (90°) and a longer side called the hypotenuse opposite to the right angle. The legs and the hyponetuse form two angles that must sum 90°, and they comply:
Where opp-leg is the opposite leg to
The rope from the top of a tree house is 85-foot long. It's the hypotenuse (L) of the triangle it forms with the ground and the tree. The angle of 45° is opposite to the height of the tree (h).