GD&T is a system of symbols, rules and definitions used to define the geometry of mechanical parts. This system is used to ensure that the products are the most economically produce through that everyone is on the same direction regarding how a feature or part is defined, produced and inspected.

• size - the overall dimensions are as specified
• form - the shapes specified must have the correct geometrical form
• fit - two parts must mate as specified
• function - the product conforms to performance specification
• reliability - the product performing its intended function under stated conditions without failure for given a period of time.
• interchangeability - the part able to replace without do any modification.