The angle system divides the circumference into 360 parts and the semicircle into 180 parts, each part is 1 degree. The semicircle of an arc system is defined as radian. Comparing the two phases, 180 degrees = radian, 1 degree = (radian/180); 30 degrees =(30/ 180) radian = radian /6, omitting ".

Radian "means 30 degrees =σ/6.