A theorem formulated by the Swiss-German mathematician and physicist Leonhard Euler (1707–83) and proved by him in 1775, which states that any displacement of a rigid body in three-dimensional space, such that one point on the body remains fixed, is equivalent to a single rotation of the body about an axis passing through the fixed point, known as the Euler pole or pole of rotation. This also means that any rotation can be described using three rotation angles, called Euler angles.