Lines, rays, segments, and angles

In plane geometry, a line is an infinite set of points extending in both directions. A line segment is the part of a line between two distinct points, called endpoints. A ray is a part of a line that starts at an endpoint and extends infinitely in one direction. An angle is the figure formed by two rays (the sides of the angle) sharing a common endpoint (the vertex).

Important properties

• A line has no endpoints and infinite length.

• A segment has two endpoints and finite length.

• A ray has one endpoint and is infinite in one direction.

• Angles are measured in degrees or radians.

• The sum of angles on a straight line is 180°.