Letters determine the gap or overlap relative to the theoretical nominal size:

The standard applies primarily to , though its principles influence fits for other geometries as well.

ISO 286 (often associated with the older DIN ISO 286 standard) provides a system for defining tolerances on linear sizes. It is divided into two main parts:

These tables provide the values for "es," "ei," "ES," and "EI" for each shaft and hole designation (a, b, c, d... z for shafts; A, B, C, D... Z for holes). For example, a "g" shaft (commonly used for sliding fits) has a specific upper deviation.