An alternate definition of radians is sometimes given as a ratio. Instead of taking the unit circle with center at the vertex of the angle θ, take any circle with center at the vertex of the angle. Then the radian measure of the angle is the ratio of the length of the subtended arc to the radius r of the circle. For instance, if the length of the arc is 3 and the radius of the circle is 2, then the radian measure is 1.5.