GD&T (Geometric Dimensioning and Tolerancing) is a particular way of communicating dimensions and tolerances for size, location, orientation, and form. In North America, the current standard is ASME Y14.5-2009, which is very similar to ISO, DIN, JIS, and other standards used outside of the US. The basic idea of GD&T:
Geometric: A way of thinking about and communicating dimensions and tolerances for a product.
Dimensioning: Calculating, recording, and communicating desired values for size, location, orientation, and form.
Tolerancing: Calculating, recording, and communicating acceptable ranges for size, location, orientation, and form.
GD&T is sort of a "self-contained" language for communicating desired and acceptable product geometries, defined by ASME (the American Society of Mechanical Engineers) in ASME Y14.5-2009. Previous editions include ASME Y14.5M-1994 and ANSI Y14.5M-1982. (Even though the standard has changed over the years from "ANSI" to "ASME" it is still considered an ANSI standard.) We offer training on the ANSI/ASME standard, but also discuss differences between the ASME and ISO standards.
To get started with GD&T, download a free reference guide (1994 version or 2009 version) to see the symbols used to control features, or enroll in our "Introduction to GD&T" course for only $20.
Also, explore our new Interactive Map of the GD&T World.