GD&T reference
GD&T Symbol Library
The fourteen geometric characteristic symbols of ASME Y14.5 / ISO 1101, grouped by tolerance type — plus the modifiers, rules, datum concepts, and general dimensioning notation behind them. Each page explains what the control does, how its tolerance zone works, how MMC/LMC modifiers apply, and how the feature is measured.
Form tolerances
Circularity (roundness)
Controls roundness of a cross-section of a cylinder, cone or sphere.
Cylindricity
Simultaneously controls roundness and straightness of a cylindrical surface.
Profile tolerances
Profile of a line
Controls the form of a line element of a surface along a specified section.
Orientation tolerances
Location tolerances
Position
Locates a feature (hole, pin, slot) relative to datum features at a basic location.
Runout tolerances
Circular runout
Limits composite variation (roundness + concentricity) in one circular cross-section.
Modifiers, rules & datums
Maximum Material Condition (MMC)
The Ⓜ modifier: the tolerance applies at maximum material, and departing from MMC grants bonus tolerance.
Least Material Condition (LMC)
The Ⓛ modifier: the tolerance applies at least material — for walls, coatings and material retention.
Regardless of Feature Size (RFS)
The default: the tolerance applies as stated at any produced size — no bonus, no symbol in the frame.
Feature Control Frame
The box that carries a geometric requirement: characteristic symbol, tolerance value, modifiers, and datum references.
Datum Feature
The actual surface labeled A/B/C on the part — and the perfect theoretical datum derived from it.
Datum Target
Points, lines and areas on rough surfaces that collectively establish a datum.
Rule #1 — Envelope Principle
The envelope principle: perfect form at MMC, with form error bounded as the size departs from MMC.
Independency (Ⓘ)
The Ⓘ modifier that removes Rule #1's envelope — size and form become independent requirements.
Envelope Requirement (Ⓔ) — ISO Only
ISO's opt-in envelope: Ⓔ restores the MMC boundary that ISO leaves off by default.
Projected Tolerance Zone (Ⓟ)
The Ⓟ modifier that projects the zone above a threaded hole, to control the fastener itself.
Unequally Disposed Profile (Ⓤ)
The Ⓤ modifier that shifts a profile zone off-center — more of the tolerance on one side of the true profile than the other.
Continuous Feature (CF)
The CF symbol that makes separated, interrupted features evaluate as one single continuous feature.
Free State (Ⓕ)
The Ⓕ modifier: the tolerance applies to the part unrestrained — no clamping, no fixturing, natural shape.
Restrained Condition Note
A text note requiring the part to be measured while restrained — held the way it will be held in service.
Tangent Plane (Ⓣ)
The Ⓣ modifier: apply the tolerance to the plane contacting the surface's high points — the contact plane, not the surface itself.
General dimensioning notation
Basic Dimension
A theoretically exact dimension, boxed on the drawing: it carries no tolerance of its own — the geometric controls applied to the same features supply all allowable variation.
Reference Dimension
A dimension repeated for information only, shown in parentheses: it contributes nothing to production or inspection because other dimensions already govern its value.
Dimension Not to Scale
A straight thick underline beneath a dimension value, stating the drawn geometry is not at the stated scale — the printed value governs, the picture does not.
Diameter (⌀)
The ⌀ symbol placed before a value states it is a diameter — the width of a circular feature through its center — and it must precede every diameter value, in dimensions and in feature control frames alike.
Radius (R)
R placed before a value states a radius: the distance from a feature's center to its surface, controlling the partial circular contours — fillets, rounds, arcs — that a diameter cannot describe.
Controlled Radius (CR)
The CR variant of a radius: the surface must both stay inside the radius tolerance zone and form a fair curve — continuous, without reversals — so the contour cannot wiggle its way through a zone it technically fits.
Square (□)
The □ symbol before a value states a square shape: the value and its tolerance apply to every side of the feature from one callout — one dimension governing two.
Dimension Origin
A small circle replacing one arrowhead of a dimension: the dimension originates from the surface the circle sits on — fixing where the measurement starts without creating a datum feature.
Depth Symbol (Depth/Deep)
The ↧ symbol before a value states it is a depth measured from the surface: how far a hole, slot or counterbore extends below the surface it starts from.
Counterbore (⌴)
The ⌴ symbol before a diameter (and optional ↧ depth) specifies a flat-bottomed enlarged hole — the recess that lets a fastener head sit flush below the surface.
Spotface
A spotface is the minimum-depth flat seat machined onto a rough or curved surface so a fastener head or washer bears on clean, flat material — dimensioned ⌴ with the SF note where the entry needs clarifying.
Countersink (⌵)
The ⌵ symbol before a diameter and an included angle specifies the conical entry that lets a flat-head fastener sit flush — the only hole callout whose angle is part of the specification.
Conical Taper
The conical taper symbol before a ratio states the rate of diameter change along a cone — (D − d)/L — replacing two end diameters with a single controlled value.
Slope
The slope symbol before a ratio states the inclination of a flat surface — the height difference between its ends divided by their separation — the flat-taper counterpart of the conical taper callout.
Arc Length
A small arc drawn over a dimension value states the value is the distance measured along the curve — the arc's true length — not the straight chord between its endpoints.
Parting Line
A diamond-marked phantom line on casting, forging and molded-part drawings showing where the mold or die halves meet — the seam the process will leave on every part unless the drawing says otherwise.
Multiple Identical Features (#X)
A numeral followed by X before a dimension — `6X ⌀8` — states how many identical features that callout governs: one note, six holes, one tolerance authority.