This British Standard gives guidance on the selection of fits given in BS1916‑1.
The recommendations apply, in particular, to fits between cylindrical parts, designated as “holes” and “shafts”, in which case the term “size” refers to the diameter of the mating parts. The recommendations are also applicable to parts other than cylindrical, in which case the term “size” refers to a length, width or other dimension.
Tables of preferred sizes are included to avoid the indiscriminate selection of dimensions, and their use is strongly recommended.