A reference angle is an angle formed by the x-axis and the terminal side of a given angle, excluding quadrantal angles. It is a helpful tool when finding the values of trigonometric functions belonging to particular angles.
There are certain tables found in calculus and trigonometry that present information in terms of reference angles. When using these tables, the sine, the cosine or the tangent of an angle whose terminal side is located in quadrant II, III or IV is determined by referring to the corresponding reference angle.
Given the way terminal angles are constructed, they are always acute and always positive. The measure of any given reference angle must therefore be between zero and 90 degrees.