Geometric Dimensioning and Tolerancing (GD&T) is a system for defining and communicating engineering tolerances. It uses a symbolic language on 2D engineering drawings and 3D solid models that explicitly describes nominal geometry and its allowable variation. It tells the manufacturing technicians what degree of accuracy and precision is needed on each controlled feature of the part. GD&T is used to define the nominal (theoretically perfect) geometry of parts and assemblies, to define the allowable variation in form and possible size of individual features, and to define the allowable variation between features.
Fundamentals of GD&T, 3rd edition by 
Ultimate GD&T Pocket Guide by 
The study of measurement is a basic requirement in any field of science and technology, most importantly in engineering and manufacturing. Since metrology is the study of measurement, it is expected to enforce, validate and verify predefined standards for traceability, accuracy, reliability, and precision. In manufacturing, the engineering drawing (including Dimensions, Tolerances, and GD&T) is the specification for the metrology process.
The Metrology Lab at Greenville Tech's CMI Campus
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