Geometric Dimensioning and Tolerancing (GD&T) is a symbolic language used in engineering drawings and computer-aided design (CAD) models to precisely communicate allowable variation in part geometry. Unlike traditional dimensioning, which primarily controls the size and location of features, GD&T adds geometric controls that define the shape, orientation, and relationship of part features with high accuracy. It ensures that manufactured parts meet functional requirements while allowing for acceptable manufacturing variations, ultimately improving quality, reducing costs, and enhancing interoperability among design, manufacturing, and inspection teams.

GD&T uses a system of standardized symbols defined by the ASME Y14.5 or ISO 1101 standards, which include feature control frames, datums, and tolerance zones. These symbols specify controls such as flatness, straightness, roundness, cylindricity, parallelism, perpendicularity, angularity, position, concentricity, symmetry, and runout. A key concept in GD&T is the datum system, which establishes a reference framework for measurement and ensures that features are accurately located relative to critical functional surfaces. For example, the position tolerance controls how much a feature's location can vary from its exact theoretical position, ensuring that parts will fit and function correctly in assemblies.