R or R represents the set of real numbers in set theory, and the real part of complex numbers is also represented by this symbol. Geometry r or r represents the radius of a circle, and the English word radius. In geometry, ∠R stands for right angle and the English word right angle. Geometry r also represents radian (an angle, 360 degrees equals radian 2 π), representing the radian of English words. Calculus uses the capital R of writing style to express Riemann integral.