Ray

One way to think of a ray is a line with one end. A ray starts at a given point and goes off in a certain direction forever, to infinity. The point where the ray starts is called (confusingly) the endpoint.

On its way to infinity it may pass through one or more other points. In the figure above, the ray starts at A and also passes through B.

A ray is one-dimensional. It has zero width. If you draw a ray with a pencil, examination with a microscope would show that the pencil mark has a measurable width. The pencil line is just a way to illustrate the idea on paper. In geometry however, a ray has no width.

A ray has no measurable length, because it goes on forever in one direction.

Drawing a ray

Naming of rays

1. By two points.
   In the figure at the top of the page, the ray would be called AB because starts at point A and passes through B on its way to infinity. Recall that points are usually labelled with single upper-case (capital) letters. There is a shorthand way of writing this: This is read as "ray AB".  The arrow over the two letters indicates it is a ray, and the arrow direction indicates that A is the point where the ray starts.
2. By a single letter.
   The ray above would be called simply "q". By convention, this is usually a single lower case (small) letter. This is normally used when the ray does not pass through another labeled point.

Coordinate Geometry

In another branch of mathematics called coordinate geometry, the points that define a ray are located on the plane using their coordinates - two numbers that show where the point is positioned.
For more on this, see Ray definition (Coordinate Geometry).

Other line topics

• Point definition
• Line definition
• Vertical lines
• Horizontal lines
• Line segment
• Midpoint of a line
• Ray
• Angle
• Opposite rays
• Intersection
• Parallel lines
• Transversal
• Line bisector
• Perpendicular bisector
• Coplanar
• Collinear points
• Plane