Types of angles

Straight angles

If you use a protractor to measure the angle of a straight line, it would measure 180 degrees (180°). We can say that the angle of a straight line is 180 degrees. We can also say that 180 degrees is a straight angle. Look at the straight angle below.

Straight angle (Image by Lfahlberg and used under this creative commons license)

Right Angles

An angle that measures 90 degrees (90°) is called a right angle. Right angles are very important in Mathematics. The corners of the rooms in your home are probably right angles. Squares and rectangles have four right angles. Look at the image of the right angle below.

Note that a small square is used to indicate a right angle.

Acute Angles

Any angle that measures less than 90 degrees (90°) but more than zero degrees (0°) is called an acute angle. Acute angles are smaller than right angles. Look at the acute angles below.

An acute angle measures less than 90 degrees. (Image by Lfahlberg and used under this creative commons license)

This acute angle measures 45 degrees. (Image by Gustavb and used under this creative commons license)

Obtuse Angles

Any angle which is greater than 90 degrees (90°) but less than 180 degrees (180°) is called an obtuse angle. Obtuse angles are larger than right angles. Look at the obtuse angles below.