Endpoint

In geometry, an endpoint is a point at the end of a line segment or ray.

Naming objects using endpoints

Many objects in geometry such as line segments, angles, polygons, etc., can be named using endpoints:

Points A and B are endpoints for the line segment below. The line segment is named by its endpoints, AB or BA.

A ray on the other hand, has only one endpoint as shown below. The ray is named using its endpoint as the first letter, AB.

An angle can be named using the common endpoint of the two rays, as shown below. Rays AB and BC share endpoint B and form an angle. This angle can be named as ∠ABC or, using just its endpoint, as ∠B.

A polygon is formed by three or more-line segments. The polygon below, called a quadrilateral, is formed by segments AB, BC, CD, and AD, called sides. Each side intersects two other sides, at each endpoint. One way to name a polygon involves listing the endpoints of the polygon in sequence; the polygon below can be named ABCD or BCDA, among other names.

Length of a line segment and endpoints

In coordinate geometry, the endpoints of a line segment can be used to determine the length of the segment using the equation:

Where (x₁, y₁) and (x₂, y₂) are the coordinates of the two endpoints.