Parallelism (∥)
Parallelism (∥) is an orientation tolerance: a surface or axis has to lie between two planes, or inside a cylinder, held parallel to a datum.
Easily confused with
How to read it
The symbol is two parallel slanted lines. A parallelism frame holds the symbol, the tolerance and at least one datum letter (Cogorno). For a surface, the zone is two parallel planes the tolerance apart, themselves parallel to the datum; for an axis it can be a cylinder (National Physical Laboratory). The datum matters: NPL’s guide notes that surface A parallel to datum B is not the same requirement as surface B parallel to datum A. ASME Y14.5-2018 groups parallelism with Perpendicularity (⟂) and Angularity (∠) as orientation controls (Berez, UNC Charlotte).
Also searched as: ∥, parallelism gd&t, parallelism symbol.
This explains what a symbol specifies on a drawing. It isn't inspection or design guidance; the drawing and the edition of the standard it cites take precedence.
Look-alikes
Flatness ⏥ also has two parallel planes, but floating free of any datum; Berez’s notes say it directly, “Parallelism is NOT flatness”, because parallelism has a datum reference. The same two strokes are geometry’s parallel sign ∥, which Unicode encodes as U+2225 PARALLEL TO and distinguishes from the double vertical line ‖ (U+2016).
Read next
- Next in GD&T symbolsPosition (⌖)Position (⌖), often called true position, controls where a feature of size such as a hole, pin or slot sits: its axis or centre plane has to lie in a zone centred on the exact location given by basic dimensions from the datums.
- The same shape in US state symbolsSouth Carolina state symbolsSouth Carolina’s state bird is the Carolina wren (1948), its flower the yellow jessamine (1924) and its tree the palmetto (1939). It has two mottoes: “Animis opibusque parati” and “Dum spiro spero”.
More GD&T symbols
The same shape elsewhere
Browse symbols drawn with lines.
Sources
- 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
- Fundamentals of Geometric Dimensioning and Tolerancing (GD&T), Part I, Jaime Berez, seminar adapted from an undergraduate mechanical engineering course, version 2.1.0, 2023, Center for Precision Metrology, University of North Carolina at Charlotte (Zenodo), Slides 32 and 34
- Geometric Dimensioning and Tolerancing for Mechanical Design: Instructors’ Guide, Gene R. Cogorno, guide to the McGraw-Hill textbook, rev. 10/12/2020, Technical Training Consultants, Chapter 6, Orientation, review answers
- Mathematical Operators: Range 2200–22FF (code chart), The Unicode Standard, Version 18.0, Unicode Consortium, U+2225
Researched, edited and approved by Rory Hansen before publishing. Last reviewed October 10, 2026. How we research
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