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...

The Beginning:  What we now call Geometric Dimensioning & Tolerancing, or "GD&T" started near the beginning of World War II.  Stanley Parker developed the concept of "True Position" and also, reportedly the first use of round tolerance zones for round holes while he worked in a torpedo factory in Scotland.  He realized that parts that had been rejected based on measurement results were actually functional, so as developed tolerancing practices to  more effectively address functional requirements of mechanical parts, he was credited with creating the first elements of GD&T.

Development of GD&T Standards:  GD&T standards started as military standards, such as US Army 30-1-7, dated April 15th, 1946, and then Mil-Std-8, dated December 30th, 1949, then the ASME (American Society of Mechanical Engineers) was the publisher for ASA (American Standards Association) Y14.5-1957.  Subsequent Y14.5 standards have been released as USASI (United...