Opposite angles, known as vertically opposite angles, are angles that are opposite to each other when two lines intersect. Vertically opposite angles are congruent, meaning they are equal in degrees of measurement. An angle measures the amount of rotation and can be expressed as degrees, radians and gradians. In some cases, the phrase "plane angle" is used to distinguish angles in a plane from angles in space.

The angle has two basic parts, two lines and an intersecting point between them. The corner where the two lines meet is called the angle. The lines that intersect to form the angles are called the arms. The angle describes the amount of turn that takes place between each arm.

Angles can be named in two ways. The first is by a lowercase letter corresponding to where the angle is located, such as angle a or angle b. It is sometimes denoted with a letter from the Greek alphabet such as alpha. An angle can also be named by the letters that define the shape of the angle, with the middle letter indicating where the angle is located.

Angles can be given positive or negative signs, depending on the direction they follow when measured. Positive angles go counter clockwise while negative angles go clockwise.