Dimensional tolerances control feature sizes, but a part can have acceptable sizes and still fail to assemble because surfaces are not flat, axes are not aligned or holes are misplaced.

Geometric dimensioning and tolerancing (GD&T) specifies acceptable variation in geometry relative to the part's function.

Datums

A datum is a theoretically exact reference derived from a real feature such as a plane, axis or point. A datum reference frame establishes the coordinate system from which related geometric requirements are interpreted.

Primary, secondary and tertiary datums progressively constrain the part's degrees of freedom during inspection or assembly.

Geometric characteristics

GD&T controls characteristics including form, orientation, location, profile and runout.

A feature-control frame identifies the characteristic, tolerance zone and any datum references needed to define the requirement.

Functional meaning

The same numerical tolerance can mean very different things depending on the shape of its tolerance zone and the references used. GD&T therefore specifies how geometry may vary, not merely how much one measured dimension may change.