If theoretical problems are under consideration, angles are not measured in degrees, but in radians (circular measure): The magnitude of an angle α is given by the length l of the arc, intercepted by the arms of the angle α on the unit circle with centre at the vertex of the angle (Fig. 2.1). We shall denote the magnitude of the angle α in circular measure again by α; sometimes, instead of α, the notation arc α° is employed, α° denoting the magnitude of the angle α expressed in degrees (in the sexagesimal system).