An angle is formed by two rays with a common endpoint. Each ray is called an arm of the angle. The common endpoint is called the vertex of the angle.

The measure of an angle is a number representing the spread of the two arms of the angle.

• Acute angle: An angle less than 90°.
• Right angle: An angle equal to 90°.
• Obtuse angle: An angle greater than 90° and less than 180°.
• Straight angle: An angle equal to 180°.
• Reflex angle: An angle greater than 180° and less than 360°.