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. Build any frame below and it is checked against the standard clause by clause, read out in plain English, and its tolerance zone drawn; then the datum reference frame, every modifier with a diagram, Rules #1 and #2, and each geometric characteristic symbol with 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.
Tolerance zone · Position
Position: the axis of this hole must lie within a cylinder Ø0.25, centred on the true position set by basic dimensions from datums A, B and C, in that order. At MMC: that is the zone when the hole is at its MMC size; as it is made larger, the zone grows by the same amount (bonus tolerance). Its surface must also not cross the virtual condition boundary, MMC size − 0.25. The part is seated on A first (primary), then B, then C.
Nothing to report: an ordinary frame.
Symbols
Every geometric characteristic, grouped by category. Concentricity and symmetry were removed from ASME Y14.5-2018 and stay current in ISO 1101:2017.
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 | Usually | Active | ISO 1101 Position |
| ◎ | Concentricity | Yes | Removed | ISO 1101 Coaxiality / concentricity (current in ISO) |
| ⌯ | Symmetry | Yes | Removed | ISO 1101 Symmetry (current in ISO) |
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 |
Datums and the datum reference frame
Datums are where measurement starts. The part is seated on its datum features in the order the frame lists them, on three mutually perpendicular planes; each one takes away the freedoms the ones before it left.
free 6 of 6
Modifiers
A modifier after the tolerance or a datum letter changes how the zone applies. Each one below with its rule and what it does to a part.
Hole Ø10.0–10.4, position Ø0.25 Ⓜ. Made at Ø10.20: Ø0.25 + 0.20 bonus = Ø0.45.
The rules
Rule #1: size controls form (the envelope principle)
A feature of size may not break a boundary of perfect form at MMC. At MMC it must be perfect; the further it is made from MMC, the more its form may vary, by exactly that amount. Nothing requires perfect form at LMC. (ASME Y14.5-2018 §5.8.1)
Rule #2: RFS and RMB are the default
With no modifier a tolerance applies regardless of feature size (RFS), and a datum feature regardless of material boundary (RMB). Ⓜ or Ⓛ must be written to get MMC or LMC (MMB or LMB on a datum). The old Ⓢ symbol adds nothing. (ASME Y14.5-2018 §5.9.1)
Virtual and resultant condition
The virtual condition is the constant boundary the size limit and the geometric tolerance make together: the size of a functional gauge. The resultant condition is the single worst boundary on the other side, once the bonus is added. (ASME Y14.5-2018 §3.70, 3.54)
ASME Y14.5-2018 and the ISO system: the key difference
Under ASME Y14.5-2018 size controls form by default (Rule #1). Under ISO 8015:2011 each requirement stands on its own (the independency principle): a size tolerance does not control form unless the envelope requirement Ⓔ is written after it, or a general note invokes it. Read the title block before reading the frame.
| Topic | ASME Y14.5-2018 | ISO GPS |
|---|---|---|
| Size and form | Envelope by default (Rule #1); Ⓘ lifts it. | Independent by default (ISO 8015:2011); Ⓔ invokes the envelope. |
| What a size tolerance checks | Local size and the mating envelope. | A two-point local size unless a modifier says otherwise (ISO 14405-1:2025). |
| No Ⓜ or Ⓛ | RFS (Rule #2). | The tolerance applies whatever the size: the same in effect. |
| ◎ and ⌯ | Removed in 2018: use position, runout or profile. | Current in ISO 1101:2017: ◎ is coaxiality for an axis, concentricity for a centre point. |
| Unequal profile | Ⓤ then the part of the zone that adds material. | UZ then the offset of the zone's centre, + out of the material, − into it. |
| Datums | Datum reference frame from datum features in order of precedence (Section 7). | Datum systems in order of precedence (ISO 5459:2024), with the association to the feature defined mathematically. |
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) and runout tolerances are measured relative to something else, so they require datums. Position almost always names them too; without any it relates the toleranced features only to each other, such as a coaxial pair or a pattern to itself. 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.