Angles (Mathematics Glossary)
What Is an Angle?
An angle is created by two rays that have a common end point, called the vertex.
The angle is also a measure of the rotation between the two rays.
Angles can be measured in degrees (°) or in radians.
How to Identify an Angle
Angles are often identified using Greek letters (α, β, γ, θ, ψ). The letter theta θ is often used.
In geometric figures, points are often labelled with capital letters. Consider the angle made between points ABC, where the vertex is A and AB and AC are the rays:
The angle can be identified as the angle A, or by ∠BAC, where the middle of the 3 letters is the vertex.
How to Measure an Angle
An angle is measured by the shortest rotation between the two rays.
Angles are often measure in degrees, denoted °. There are 360 degrees in a full rotation.
Angles can also be measured in radians. There are 2π radians in a full rotation.
Angles can be measured directly using a protractor:
Angles can also be calculated using geometry and trigonometry.
Types of Angles
The different types of angles are:
Type of Angle  Description  

Acute  Less than 90°  
Right  90°  
Obtuse  Greater than 90° but less than 180°  
Straight  180°  
Reflex  Greater than 180°  
Full  360° 
Curriculum
Interactive Test
showHere's a second test on angles.
Here's a third test on angles.
Note
Parts of an Angle
An angle is created by two rays meeting at a common endpoint, the vertex.

A ray is a line which starts from a point.

A vertex is the point where the two rays meet.
What's in a Name?
Angle comes from the Latin word 'angulus' meaning "corner".
It has the same root as the word "ankle", because the foot forms a corner with the leg.