Other standards, such as those from the International Organization for Standardization (ISO) describe a different system which has very different interpretation rules (see GPS&V). One such standard is American Society of Mechanical Engineers (ASME) Y14.5. There are several standards available worldwide that describe the symbols and define the rules used in GD&T. Dimensional specifications define the nominal, as-modeled or as-intended geometry, while tolerance specifications define the allowable physical variation of individual features of a part or assembly.
GD&T is used to define the nominal (theoretically perfect) geometry of parts and assemblies, the allowable variation in size, form, orientation, and location of individual features, and how features may vary in relation to one another such that a component is considered satisfactory for its intended use. Geometric dimensioning and tolerancing ( GD&T) is a system for defining and communicating engineering tolerances via a symbolic language on engineering drawings and computer-generated 3D models that describes a physical object's nominal geometry and the permissible variation thereof. System for defining and representing engineering tolerances Example of geometric dimensioning and tolerancing
