Symmetry (⌯)
Symmetry (⌯) required the median points between two opposed surfaces, such as the two sides of a slot, to lie between two parallel planes centred on a datum. ASME Y14.5-2018 removed it from the US standard; ISO drawings still use it.
How to read it
The symbol is three stacked horizontal lines, the middle one longest. ASME Y14.5-2009 defined symmetry, as Mitutoyo’s Jim Salsbury quotes it, as the condition where the median points of all opposed elements of two or more feature surfaces are congruent with a datum axis or center plane. Cogorno’s guide describes the zone as two parallel planes evenly disposed about the datum’s centre plane or axis, applying only regardless of feature size. Unicode encodes it among its drafting symbols as ⌯ (U+232F) SYMMETRY.
Also searched as: ⌯, symmetry gd&t, symmetry 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.
Removed from ASME Y14.5 in 2018
ASME Y14.5-2018 removed symmetry together with Concentricity (◎), term, symbol and concept (Salsbury, quoting paragraph A-8.4). NIST and UNC Charlotte’s seminar notes say it went for the same reasons, a complex and often misunderstood definition, and that Position (⌖) now controls the location of a feature’s centre line or centre plane. Cogorno’s guide shows position locating a feature of size symmetrically about a datum feature of size, with maximum material modifiers.
Symmetry on ISO drawings
ISO’s system keeps a symmetry tolerance. The National Physical Laboratory’s guide reads a symmetry frame on a slot as a zone between two parallel planes, 0.175 mm apart in its example, set symmetrically about the datum median plane. Keyence’s current list of symbols, which marks the symbols that are ASME only, lists symmetry as a location tolerance without that mark.
Read next
- Next in GD&T symbolsCircular runout (↗)Circular runout (↗) limits how much each circular element of a surface varies as the part turns a full revolution about a datum axis. It needs a datum axis.
- The same shape in Logic gate symbolsBuffer gateA buffer passes its input straight through: 1 gives 1 and 0 gives 0. Its symbol is a triangle with no circle; a buffer strengthens a signal’s drive without changing its logic.
More GD&T symbols
The same shape elsewhere
Browse symbols drawn with lines.
Sources
- GD&T and the new ASME Y14.5-2018, Jim Salsbury, Mitutoyo Institute of Metrology, presentation, 2019, Mitutoyo America Corporation, Slides on ASME Y14.5-2018 changes
- Fundamentals of Geometric Dimensioning and Tolerancing (GD&T), Part II, Jaime Berez (UNC Charlotte) and Maxwell Praniewicz (NIST), seminar slides, National Institute of Standards and Technology, Slide 5
- 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, Appendix A and Chapter 8, review answers
- 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, Symmetry
- GD&T Symbols, GD&T Fundamentals, KEYENCE America, Location Tolerance
- Miscellaneous Technical, Range: 2300–23FF (code chart), The Unicode Standard, Unicode Consortium, Drafting symbols, U+232F
Researched, edited and approved by Rory Hansen before publishing. Last reviewed October 9, 2026. How we research
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