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There are different ways to denote an angle and all involve the symbol $∠$

in front of the name. One way is to name an angle by its vertex alone. Alternatively, it can be named by using all three points that make up the angle. In this case, the vertex is always in the middle of the name. Additionally, angles within a diagram can be denoted with numbers or lowercase Greek letters.

Using the Vertex | Using the Vertex and One Point on Each Ray | Using a Number | Using Greek Letters |
---|---|---|---|

$∠B$ | $∠ABC$ or $∠CBA$ | $∠1$ | $∠α$ or $∠β$ or $∠θ$ |

The *measure of an angle*, denoted by $m∠,$ is the number of degrees between the rays. It is found by applying the Protractor Postulate. When two angles have the same measure, they are said to be congruent.

An angle divides the plane into two parts.

- The region between the sides, or
interior

of the angle - The region outside the sides, or
exterior

of the angle

Notice that the interior of the angle is the region for which the angle measure is less than $180_{∘}.$