
A designer often starts by creating an ideal assembly, where all the parts fit together with optimal tightnesses and clearances. He will have to convey to each part's manufacturer the ideal sizes and shapes, or nominal dimensions of all the par't surfaces. If multiple copies of a part will be made, the desinger must recognize it is impossible to make them all identical. Every manufacturing process has unavoidable variations that impart corresponding variations to the manufactured parts. The designer must analyze his entire assembly and assess for each surface of each part how much variation can be allowed in size, form, orientation and location.Then in addition to the ideal part geometry, he must communicate to the manufacturer the calculated magnitude of variation or tolerance each characteristic can have and still contribute to a workable assembly.
Geometric Dimensioning and Toleracing (GD&T) is a language for communicating engineering design specifications. GD&T includes all the symbols, definitions, mathematical formulae, and application rules necessary to embody a viable engineering language. As its name implies, it conveys both the nominal dimensions (ideal geometry), and the tolerances for a part. Since GD&T is expressed using line drawings, symbols, and Arabic numerals, people everywhere can read, write, and understand it regardless of the native tongues. It is now the predominant language used worldwide as well as the standard language approved by the American Society of Mechancial Enginees (ASME), the American National Standards Institute (ANSI), and the United States Department of Defense (DoD).
Finally, GD&T can only express what a surface shall be. It is incapable of specifying manufacturing processes for making it so. Likewise, there is no vocabulary in GD&T for specifying inspection or gaging methods. To summarize, GD&T is the language that designers use to translate design requirements into measurable specifications. 
