**The protractor postulate states that the measurement of an angle between two rays can be designated as a unique number, and this number would be between 0 and 180 degrees.** This postulate enables the use of a protractor to measure angles.

A postulate, also known as an axiom, is a starting point in reasoning. It is a premise assumed to be true so that other facts can be deduced. The concept of angle measurement is one of the most important in geometry, especially in trigonometry, which is primarily concerned with the measurement of angles. Though the protractor postulate only stipulates the 180 degrees of a half-circle, a protractor can be used to measure any of the 360 degrees in which a circle is divided. When the angle does not match a whole degree, the degrees are divided into 60 equal parts called minutes and the minutes into 60 equal parts called seconds.

Protractors are usually made of transparent glass or plastic. Many protractors have half-discs which measure 180 degrees, while others are full discs that measure a full 360 degrees. Bevel protractors have pivoting arms for measuring and marking angles. These are typically used by architects, engineers and other designers. A protractor compass is a navigational device that uses angular measurement for orienteering.