What Are the Subsets of a Line in Geometry?

By Amy Harris
A geometry class concept.

In geometry, a line is a perfectly straight one-dimensional figure extending infinitely in both directions. There are two subsets, or subcategories, of lines in geometry: line segments and rays.

Line Segments

A line segment is a piece of a line that has two distinct endpoints. Because of these endpoints, unlike a line, a line segment doesn't extend infinitely. Rather, it is finite, with a measurable length.


A ray is essentially a hybrid between a line and a line segment. A ray has exactly one endpoint -- called its origin -- and extends infinitely in the other direction. Like lines, rays are infinite and therefore immeasurable. Rays may sometimes be referred to as half-lines.

About the Author

Based in western New York, Amy Harris began writing for Demand Media and Great Lakes Brewing News in 2010. Harris holds a Bachelor of Science in Mathematics from Penn State University; she taught high school math for several years and has also worked in the field of instructional design.