Tangent plane (Ⓣ)
Ⓣ, a capital T in a circle, after the tolerance in an orientation frame means the tolerance applies to a plane resting on the high points of the surface, not to every point on it. It is an ASME symbol with no ISO counterpart.
Easily confused with
How to read it
Cogorno defines a tangent plane as a plane that contacts the high points of the specified surface, and places the circled T in the frame after the tolerance. With it, a surface whose irregularities go beyond, say, a Perpendicularity (⟂) tolerance is still in tolerance if a precision plane resting on its high points falls inside the zone. The modifier keeps the tighter orientation tolerance on the surface while its flatness is left to the size limits (Rule #1), and it can be applied to any orientation control of a flat surface (Cogorno). Keyence’s note puts the same idea as the frame specifying the convex side of the surface, not the concave.
Also searched as: Ⓣ, tangent plane gd&t, tangent plane 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.
Differences by standard
Keyence’s list of symbols marks the tangent plane “ASME only”, and its comparison of the two standards lists the contact plane among ASME symbols with no ISO equivalent.
Look-alikes
Other circled letters are modifiers too: Ⓜ, Ⓛ, Ⓟ and Ⓕ. Unicode encodes this one as Ⓣ (U+24C9) CIRCLED LATIN CAPITAL LETTER T.
Read next
- Next in GD&T symbolsStraightness (⏤)Straightness (⏤) is a form tolerance. A line on the surface, or the axis of the feature, may depart from a perfect straight line only by the amount given in the frame, and no datum is referenced.
- The same shape in Jewelry marks925 (sterling silver)The metal is sterling silver: at least 925 parts in every 1,000 are pure silver.
More GD&T symbols
The same shape elsewhere
Browse symbols drawn with circles or letters and numbers.
Sources
- The Tangent Plane Applied to Perpendicularity, Gene R. Cogorno, after Geometric Dimensioning and Tolerancing for Mechanical Design, 2nd ed., McGraw-Hill, 2011, p. 91, Technical Training Consultants, Definition and Figure 6-3
- 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
- GD&T Symbols, GD&T Fundamentals, KEYENCE America, Associated Symbols
- International Industrial Standards and GD&T, GD&T Fundamentals, KEYENCE America, Differences in Symbols
- Enclosed Alphanumerics: Range 2460–24FF (code chart), The Unicode Standard, Version 18.0, Unicode Consortium, U+24C9
Researched, edited and approved by Rory Hansen before publishing. Last reviewed October 9, 2026. How we research
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