Line, ray and segment notation (AB)
A mark over two letters says which part of a line is meant: a plain bar over AB is the segment from A to B, an arrow pointing right is the ray from A through B, and an arrow at each end is the whole line through A and B.
Easily confused with
How to read it
OpenStax’s Contemporary Mathematics sets the three out in a table. AB with a line above it is the line segment from A to B, with two endpoints. AB with an arrow above it is a ray: it starts at the first letter and goes on for ever through the second, so the ray AB and the ray BA point opposite ways. AB with a double-headed arrow is the line through A and B, going on indefinitely in both directions. OpenStax adds a fourth, an arrow with a small open circle at its start, for a half-line that leaves out its starting point. Henry Africk’s Elementary College Geometry (CUNY) uses the same arrows for a line and a ray.
Also searched as: AB, arrow over AB, bar over AB, line segment symbol, line symbol, overline, ray symbol.
AB with no mark
Books differ on the bare letters. Africk writes the segment as plain AB and notes that some textbooks use the bar over AB instead; OpenStax always puts the bar on. Read the text’s own definitions to tell whether a bare AB means the segment or the line.
Look-alikes
A bar over a single letter is something else: OpenStax’s Introductory Statistics uses x with a bar over it, “x bar”, for the sample mean. An arrow over a single letter is a vector: in OpenStax’s University Physics, a letter with an arrow above it denotes a vector. A curve over two letters is an Arc symbol (⌒) of a circle.
Read next
- Next in Geometry symbolsCongruent symbol (≅)≅ means “is congruent to”: two figures are congruent when they are the same shape and the same size, so one could be laid exactly on the other. △ABC ≅ △DEF says the two triangles are exact copies.
- The same shape in Value stream mapping symbolsFIFO laneA lane between two processes that holds a limited number of parts in the order they arrived, first in, first out. The icon is two parallel lines with FIFO written between them, often with the maximum quantity.
More geometry symbols
The same shape elsewhere
Browse symbols drawn with arrows, letters and numbers or lines.
Sources
- Contemporary Mathematics, 10.1 Points, Lines, and Planes, OpenStax (Rice University), Basic geometric symbols for points and lines (Figure 10.3); Example 10.1
- Elementary College Geometry, 1.1 Lines, Henry Africk, Elementary College Geometry (CUNY New York City College of Technology, 2021), CUNY Academic Works, via LibreTexts, Lines
- Earliest Uses of Symbols from Geometry, Jeff Miller, Earliest Uses of Various Mathematical Symbols, MacTutor History of Mathematics, University of St Andrews, Line segment
- University Physics Volume 1, 2.1 Scalars and Vectors, OpenStax (Rice University), Scalars and vectors
- Introductory Statistics 2e, 2.5 Measures of the Center of the Data, OpenStax (Rice University), Measures of the center of the data
Researched, edited and approved by Rory Hansen before publishing. Last reviewed October 9, 2026. How we research
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