Parallel symbol (∥)
∥ means “is parallel to”: the two lines run in the same direction and never meet. AB ∥ CD says line AB is parallel to line CD.
Easily confused with
How to read it
OpenStax’s Contemporary Mathematics defines parallel lines as lines in the same plane that never intersect, so the distance between them stays the same, and writes l₁ ∥ l₂. Henry Africk’s Elementary College Geometry (CUNY) gives the same definition and sign. On a diagram, parallel lines are marked instead with small matching arrowheads drawn on each line (Africk). Unicode encodes the sign as PARALLEL TO (U+2225) and the crossed-out form ∦ (U+2226), “not parallel to”.
Also searched as: ∥, ∦, double vertical line, not parallel, parallel lines symbol, parallel sign.
Look-alikes
Unicode separates ∥ from two look-alikes, the double vertical line ‖ (U+2016) and the Latin letter lateral click ǁ (U+01C1). Africk prints the sign as two keyboard pipes, ||.
On engineering drawings
In a feature control frame, two slanted strokes are the parallelism tolerance. The UK National Physical Laboratory’s guide defines its zone as two planes or two lines parallel to a datum, which the surface or axis must lie between. In geometry, ∥ is an exact relation between two lines.
Read next
- Next in Geometry symbolsAngle symbol (∠)∠ means “angle”. ∠ABC is the angle at B formed by the lines from B to A and from B to C; the middle letter is always the corner (the vertex).
- The same shape in Dashboard warning lightsEV plug-in indicatorAn information light that shows a charging cable is plugged into the car. Nissan calls it the plug-in indicator light; Hyundai calls it the charging connector indicator light and Kia the charging indicator light.
More geometry symbols
The same shape elsewhere
Browse symbols drawn with lines.
Sources
- Contemporary Mathematics, 10.1 Points, Lines, and Planes, OpenStax (Rice University), Parallel and perpendicular lines
- Elementary College Geometry, 1.4 Parallel Lines, Henry Africk, Elementary College Geometry (CUNY New York City College of Technology, 2021), CUNY Academic Works, via LibreTexts, Parallel lines
- Earliest Uses of Symbols from Geometry, Jeff Miller, Earliest Uses of Various Mathematical Symbols, MacTutor History of Mathematics, University of St Andrews, Parallelism
- Fundamental Good Practice in the Design and Interpretation of Engineering Drawings for Measurement Processes, Measurement Good Practice Guide No. 79, David Flack and Keith Bevan, 2005, National Physical Laboratory (UK), Chapter 4, Tolerances of orientation
- Mathematical Operators: Range 2200–22FF (code chart), The Unicode Standard, Version 18.0, Unicode Consortium, Relations, 2225–2226
Researched, edited and approved by Rory Hansen before publishing. Last reviewed October 9, 2026. How we research
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