Concept

An angle is the figure formed by two rays, called the sides of the angle, sharing a common endpoint, called the vertex of the angle.
The sides of an angle are two rays that share a common endpoint called the vertex of the angle.
The vertex of an angle is the point where the two sides of an angle meet.

A protractor is used to measure and draw angles. There are two sets of degrees along the edge of the protractor: an inner and outer scale. Both scales go from 0 to 180 degrees, but they run in opposite directions. Each mark on the protractor is 1degree.
The baseline, also known as the zero edge, has a small circle at its midpoint. This is the center mark or origin point. 

Rule

When using a protractor, place it on one line of the angle, lining up the center mark with the vertex (the point at which the two lines meet). 
Ensure that one of the zero edges (inner or outer scale) lines up with one line of the angle and that the other line crosses the protractor's scale. 
Read along the scale (inner or outer) starting from the zero where the first line was placed. Where the other line crosses, the scale will be the angle.

Example

What is the degree measure of the given angle?

Solution

1. On the given protractor, one line is lined up with zero on the outer scale.  

2. Read along the outer scale, starting from the zero where the first line is placed. Where the other line crosses, the scale is the angle.

Answer: 100°

Pre-requisite Skills

Measuring Angles (4.10.1)