You can measure angle sizes by using more than one scale. The *degree scale* is probably the most well‐known scale, although the radian scale is equally as popular and useful. Although most applications deal only in one of these two scales, it is important to understand their differences and how to convert from one to the other.

You can define trig functions by using a unit circle, a circle with a radius of 1. As a point revolves around the circle, its distance from the x‐axis is defined as the sine, and its distance from the y‐axis is defined as the cosine. These definitions match the previous definitions in terms of a right triangle. The graphs of trigonometric functions are used to visually represent their behavior.