Geometric Dimensioning & Tolerancing, or "GD&T", is a standardized language of symbols that define the allowable type and amount of geometric variation that a mechanical part may have.  A "mechanical part" is any physical part.  It could be a camshaft, or an artificial hip component, or it could be a printed circuit board, especially if that circuit board has something like an encoder reader on it.  Any part that must be dimensionally "correct enough" to function is a candidate for GD&T.  Deciding whether GD&T is applied or not on a given part will be discussed in an upcoming post.

The geometric variation can be the size, form (shape), orientation, or location of any feature on a part, such as a planar surface, or a cylindrical hole, or some irregular or compound curved feature.  The complication in specifying tolerances to control these things is often blamed on the GD&T language itself, but it comes from mother nature, rather than from...