GD&T, short for geometric dimensioning and tolerancing, is a symbolic language on engineering drawings that controls the form, orientation and location of features, not just their size. Each control sits in a feature control frame that names the characteristic, such as flatness or position, the size of the tolerance zone and the datums it is measured from. Datums are reference features, often a flat face and two edges or holes, that fix how the part is held for inspection. In the US, GD&T follows ASME Y14.5; ISO publishes its own system.
Example: A plate has four bolt holes. Instead of plus-or-minus dimensions from an edge, the drawing gives each hole a position tolerance of 0.2 mm diameter relative to datums A, B and C: the bottom face and two edges. Inspection holds the part on those datums and checks each hole against that zone.
Plus-or-minus dimensions alone leave room for argument about how a part should be measured and can reject parts that would fit. GD&T states the design intent once, so design, machining and inspection all work to the same rule and fewer good parts are scrapped.
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