A protractor is a simple measuring instrument that is used to measure angles. A common protractor is in the shape of a semicircle with an inner scale and an outer scale and with markings from 0° to 180° on it. Architects and designers use a more precise protractor called an angle protractor tool which gives more accurate measures. Protractors have been in use since the seventeenth century. Later by the eighteenth century, protractors were widely used in geometry and mathematics.

Suppose we have an angle to measure say, ∠ABC. Follow the steps given below to measure the angle.

1. Step 1. Put the protractor above the line BC such that the midpoint of the protractor is at point B.
2. Step 2. Adjust the protractor in such a way that BC is parallel to the straight-edge of the protractor.
3. Step 3. The protractor has two ‘scales’ marked from 0 to 180 degrees on both ends. Take the reading where BC coincides with the 0 degree.
4. Step 4. Now from 0 degrees, check the ray BA coincides with the curved edges of the protractor. This reading gives us the measure of angle ABC.

A 360° protractor is circular in shape and has readings up to 360° in the clockwise direction. It is commonly used in the field of architecture, mechanical drawings, and meteorology.

• Angles are measured in degrees or radians.
• A right angle is ¼ revolution and a straight angle is ½ a revolution.
• A straight angle forms a straight line, and both its arms coincide with the straight edges of the protractor.
• When the sum of the measures of two angles is that of a right angle, then each one of them is 45 degrees.
• When the sum of the measures of two angles is that of a straight angle and if one of them is acute, then the other should be an obtuse angle.