Perpendicularity (⟂)
Perpendicularity (⟂), also called squareness, is an orientation tolerance: a surface, axis or centre plane meant to be at 90° to a datum has to lie in a zone set exactly square to it.
Easily confused with
- Top and bottom (⊤, ⊥)⊤ (top) means “true” and ⊥ (bottom) means “false” in logic: ⊤ is a statement that is always true, ⊥ one that is always false, a contradiction.
- Perpendicular symbol (⊥)⊥ means “is perpendicular to”: the two lines meet at a right angle, 90°. AB ⊥ CD says line AB is perpendicular to line CD.
How to read it
The symbol is a vertical line standing on a horizontal one. For a surface, the zone is two parallel planes the tolerance apart, perpendicular to the datum plane (Cogorno; National Physical Laboratory); for an axis it can be a cylinder (NPL), shown by a Ø before the tolerance (Cogorno). The frame names the datum it is square to, and NPL’s guide points out that surface A perpendicular to datum B is a different requirement from surface B perpendicular to datum A. ASME Y14.5-2018 makes it one of three orientation controls with Angularity (∠) and Parallelism (∥) (Berez, UNC Charlotte). A circled T after the tolerance applies it to a plane resting on the surface’s high points rather than to every point (Cogorno). NIST’s seminar notes call the same control squareness.
Also searched as: ⟂, perpendicularity gd&t, perpendicularity symbol, squareness 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
Unicode encodes the symbol as ⟂ (U+27C2) PERPENDICULAR, “orthogonal to”, the perpendicular sign of geometry, and cross-refers it to ⊥ (U+22A5) UP TACK, the same shape used in logic and mathematics. On a drawing, Angularity (∠) ∠ covers any angle other than 90°.
Read next
- Next in GD&T symbolsParallelism (∥)Parallelism (∥) is an orientation tolerance: a surface or axis has to lie between two planes, or inside a cylinder, held parallel to a datum.
- The same shape in European flagsFlag of GreeceGreece’s flag is nine horizontal stripes, five blue and four white, with a white cross on a blue square in the top corner by the pole. Law 851/1978 fixes the design but gives it no meaning; readings of the nine stripes, such as the nine syllables of “Freedom or Death”, are popular tradition.
More GD&T symbols
The same shape elsewhere
Browse symbols drawn with crosses or 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 27 and 32
- 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
- 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 21
- Miscellaneous Mathematical Symbols-A: Range 27C0–27EF (code chart), The Unicode Standard, Version 18.0, Unicode Consortium, U+27C2
Researched, edited and approved by Rory Hansen before publishing. Last reviewed October 10, 2026. How we research
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