Tangent Plane (Ⓣ)

The Ⓣ modifier: apply the tolerance to the plane contacting the surface's high points — the contact plane, not the surface itself.

Definition

The tangent-plane modifier Ⓣ — the letter T in a circle, placed after the tolerance value — changes what a tolerance applies to: not the actual surface, but its tangent plane, the theoretically perfect plane contacting the surface at its high points. The mating part never touches the whole surface anyway; it touches the high points. So a parallelism 0.1 Ⓣ A callout requires the contact plane to be parallel to datum A within 0.1, while the surface's local waviness between the high points is unconstrained by that callout.

Why the contact plane is what matters

Fixtures, machine tables, and mounting faces all meet the part the same way: at a few high spots. If a cover plate bolts against a housing face, the leak path and the load path run through the contact plane — the surface's fine waviness changes little. An ordinary parallelism callout spends tolerance limiting that waviness, which the function does not care about, and thereby forces the surface to be better than it needs to be. Ⓣ redirects the tolerance to the effective geometry: keep the plane the part actually presents to the world within limits, and let the texture between the high points take whatever form the process produces.

Where Ⓣ may appear

The modifier belongs to the orientation family — parallelism, perpendicularity, angularity applied to a planar surface — and Y14.5-2018 also clarifies its use with profile and total runout where a tangent-plane interpretation is meaningful. It presumes a surface whose high points define a plane, so it applies to nominally flat surfaces; it is not a tool for cylindrical features (where the analogous concept is a tangent cylinder pair, handled by other means). ASME Y14.5-2009 introduced Ⓣ as a formal symbol and 2018 retains it — this is a long-standing modifier, not an experimental one, though it sees far less use than Ⓜ or Ⓛ.

Reading a feature control frame

∥ 0.1 Ⓣ A reads as: parallelism of the TANGENT PLANE, zone 0.1 wide, relative to datum A. The Ⓣ sits after the tolerance value in the usual modifier slot. Read the frame with the modifier in mind: without Ⓣ the callout bounds the entire surface; with Ⓣ it bounds only the plane through the high points, and the surface itself may wave beyond 0.1 between them.

The trade-off

• Mating behavior is contact — when the function lives at the high points, tolerancing the plane is the honest specification
• The surface gains freedom — expect more visible waviness; unacceptable for appearance surfaces regardless of function
• Inspection is subtler — the evaluator must establish the actual tangent plane, not average the surface
• Rule #1 still applies to features of size — Ⓣ modifies a geometric control, it does not remove the size envelope