GD&T symbols & modifiers

A feature control frame reads left to right: symbol, tolerance zone, then datums. ⌖ Ø0.25 Ⓜ | A | B | C means: position tolerance, a Ø0.25 cylindrical zone that grows with bonus tolerance at maximum material condition, established from datums A, B, then C in that order. This page lists each geometric characteristic symbol — form, orientation, location, profile, and runout — with whether it requires a datum, which modifiers apply, and its ISO 1101 counterpart.

Unless a modifier says otherwise, a geometric tolerance applies regardless of feature size — that is the default, so the historic Ⓢ symbol adds no meaning under current practice. Ⓜ (maximum material) and Ⓛ (least material) change how much tolerance is available as the feature departs from its size limit.

form

Symbol	Name	Datum required	ASME 2018	ISO equivalent
⏤	Straightness	No	Active	ISO 1101 Straightness
⏥	Flatness	No	Active	ISO 1101 Flatness
○	Circularity	No	Active	ISO 1101 Roundness
⌭	Cylindricity	No	Active	ISO 1101 Cylindricity

orientation

Symbol	Name	Datum required	ASME 2018	ISO equivalent
⟂	Perpendicularity	Yes	Active	ISO 1101 Perpendicularity
∠	Angularity	Yes	Active	ISO 1101 Angularity
∥	Parallelism	Yes	Active	ISO 1101 Parallelism

location

Symbol	Name	Datum required	ASME 2018	ISO equivalent
⌖	Position	Yes	Active	ISO 1101 Position
◎	Concentricity	Yes	Removed	ISO 1101 Concentricity
⌯	Symmetry	Yes	Removed	ISO 1101 Symmetry

profile

Symbol	Name	Datum required	ASME 2018	ISO equivalent
⌓	Profile of a Surface	Optional	Active	ISO 1101 Profile of a Surface
⌒	Profile of a Line	Optional	Active	ISO 1101 Profile of a Line

runout

Symbol	Name	Datum required	ASME 2018	ISO equivalent
↗	Circular Runout	Yes	Active	ISO 1101 Circular Runout
⌰	Total Runout	Yes	Active	ISO 1101 Total Runout

Modifiers

Symbol	Name	Standard	Status
Ⓜ	Maximum Material Condition	both	Active
Ⓛ	Least Material Condition	both	Active
Ⓢ	Regardless of Feature Size	asme	Symbol retired (ASME Y14.5M-1994) — RFS is the default material condition; Y14.5M-1994 retired the displayed Ⓢ symbol. Appears only on legacy drawings.
Ⓕ	Free State	both	Active
Ⓟ	Projected Tolerance Zone	both	Active
Ⓣ	Tangent Plane	asme	Active
Ⓤ	Unequally Disposed	asme	Active
Ⓡ	Reciprocity Requirement	iso	Active
CF	Continuous Feature	asme	Active
ST	Statistical Tolerance	asme	Active
Δ	Dynamic Profile	asme	Active
CZ	Combined Zone	iso	Active
SZ	Separate Zones	iso	Active
UZ	Unequally Disposed Tolerance Zone	iso	Active
OZ	Offset Zone	iso	Active
CT	Common Tolerance	iso	Active
SIM	Simultaneous Requirement	iso	Active
Ⓔ	Envelope Requirement	iso	Active

Which GD&T symbols need a datum?

The form tolerances — straightness, flatness, circularity, cylindricity — control a feature against itself and never take a datum reference. Orientation (perpendicularity, parallelism, angularity), location (position, concentricity/symmetry where still used), and runout tolerances are measured relative to something else, so they require datums. Profile can work either way: with datums it locates the surface; without, it controls form only.

ASME Y14.5 or ISO 1101 — which applies?

The drawing tells you: a title-block or general note names the governing standard, and the interpretation rules differ in places (default tolerance zones, datum precedence details, symbol sets). Aerospace drawings in the US supply chain are predominantly ASME Y14.5; European primes commonly reference the ISO GPS system built around ISO 1101. The table flags symbols whose meaning or availability differs between the two.