In geometry, two lines or planes are said to be perpendicular if they intersect at a right angle, which is an angle of 90 degrees. The concept of perpendicularity is fundamental to many areas of mathematics, including algebra, calculus, and especially geometry, where it is used to analyze and construct a wide range of geometric shapes.

For example, in the Cartesian coordinate system, the x-axis and y-axis are perpendicular to each other. Similarly, the edges of a square or rectangle are perpendicular to each other, as are the faces of a cube or rectangular prism.

The symbol for perpendicularity is ⊥. For example, if you have two lines, a and b, you can write a ⊥ b to mean “line a is perpendicular to line b”.

An angle is a measure of rotation or turning and is usually measured in degrees or radians. In geometry, an angle is the figure formed by two rays, called the sides of the angle, sharing a common endpoint, called the vertex. The size of an angle is determined by the amount of rotation from one ray to the other.