* The GD&T system uses a series of symbols and codes on drawings to communicate the characteristics of critical features that need to be produced. This system, was first developed in the 1940s in Scotland and has developed into a detailed set of standards issued by both ANSI and ISO. Geometric tolerancing is used to universally define the form, orientation, and location of part features on drawings.

* What is GD&T?

Basically it is a system that replaces words with symbols, simplifying drawings. Imagine a series of parts are required for a final assembly. Each part will be engineered to perform a function and, most importantly, to assemble with a mating part.The term tolerance could mean, 'allowable deviation'. A tolerance is the amount of deviation from the exact size allowed on a part. Any part within the tolerance zone will still be functional. So why are tolerances used on part drawings? We are all aware that we cannot manufacture anything to an exact size. This is due to variations in machining, tools and our own ability.
* Therefore a drafter or designer is to set part tolerances that are large enough to keep manufacturing costs down and close enough to ensure that all parts will assemble with the mating part, without resorting to a big hammer. It is easy for the designer to use a close tolerance on features to reduce part dimension variables; however the part cost increases dramatically as tolerances are reduced.

*Why Use GD & T

* Repeatability and Interchangeability of parts.
* Cost effectiveness of machined part.
* Simplicity in interpretation of detail.