A drawing must specify not only the nominal geometry of a part but how much variation is acceptable.

Dimensions

Dimensions define sizes and locations of features. Good dimensioning communicates each requirement clearly without unnecessary duplication or dependence on measuring from a printed scale.

Tolerances

Manufacturing cannot produce an exact mathematical dimension. A tolerance defines the permitted range around a nominal size.

For example,

$$20.00\pm0.05\ \mathrm{mm}$$

allows sizes from $19.95$ to $20.05$ mm.

Fits

When a shaft and hole must assemble, their dimensional tolerance zones determine whether the result has clearance, interference or a transition between the two.

Functional tolerancing

Tighter tolerances generally increase manufacturing and inspection cost. A useful tolerance is therefore not the smallest number possible, but the largest variation that still guarantees the intended function and interchangeability.

Size tolerances control dimensions such as length or diameter; geometric tolerances extend this idea to shape, orientation and location.