The short answer: Geometric Dimensioning and Tolerancing (GD&T) is a graphic language used on engineering drawings to communicate allowable part feature variation – or tolerances – to manufacturing and quality personnel. It is based on how parts fit and function in their assemblies. This method of establishing tolerances enables improved design, reduced manufacturing costs and streamlined quality inspection.
GD&T can be applied to one-off designs as well as high volume manufacturing. It can improve the design of any product type including sheet metal, castings and mouldings, fabricated and machined components, tooling, fixtures and gauges. GD&T allows you to specify exactly what you need for parts to function, relaxing tolerances, and controlling variations in size, form, orientation, location and profile. Traditional co-ordinate or plus-minus tolerancing simply does not have the tools to communicate these requirements. GD&T ensures part interchangeability, allows the selection of less costly manufacturing and assembly methods, improves communication, and by helping designers understand the effects of manufacturing variation, improves design.