GD&T (Geometric Dimensioning and Tolerancing) is a symbolic language for engineering drawings that defines the allowable form, orientation, location and runout of part features, standardized by ISO 1101 and ASME Y14.5.

How many geometric characteristic symbols are there?

There are 14 geometric characteristics in the classic set: straightness, flatness, circularity, cylindricity, profile of a line, profile of a surface, perpendicularity, parallelism, angularity, position, concentricity, symmetry, circular runout and total runout. ASME Y14.5-2018 removed concentricity and symmetry, leaving 12.

Which GD&T symbols require a datum?

Form controls (straightness, flatness, circularity, cylindricity) need no datum. Orientation, location and runout controls always reference one or more datums. Profile controls may be used with or without a datum.

What does the Ⓜ (MMC) modifier mean?

Ⓜ denotes Maximum Material Condition: the geometric tolerance applies when the feature contains the maximum amount of material, and additional bonus tolerance becomes available as the feature departs from MMC.

What is the difference between ISO 1101 and ASME Y14.5?

Both define the same core symbols, but ISO uses the default Regardless of Feature Size rule differently, structures datum references and modifiers slightly differently, and ASME Y14.5-2018 dropped concentricity and symmetry in favor of position and runout. Always follow the standard cited on the drawing.

Reference

GD&T Symbols

All 14 geometric characteristic symbols of ISO 1101 / ASME Y14.5 — with category, datum requirement and a plain-language description. Plus the modifier symbols. Click a symbol to copy it.