# Radian Angle System

This figures illustrates the radian angle system annotated using the circle constant (tau). See this figure for radians annotated with (pi).

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Radians are a unit that measure angle using the radius of a circle. One radian is equal to the amount of rotation required to travel the length of one radius along the circumference of the circle.

This figure illustrates the radian Angle System annotated with the constant π (pi).