The chromosomal locus of a gene might be written "6p21.3". Here the 6 is the chromosome number. The letter p indicates that the position is on the chromosome's short arm (p for petit in French); q indicates the long arm. The numbers following the letter represent the position on the arm: band 2, sub-band 1, sub-sub-band 3. The bands are visible under a microscope when the chromosome is suitably stained. Each of the bands is numbered, beginning with 1 for the band nearest the centromere. Sub-bands and sub-sub-bands are visible at higher resolution.

A range of locations is specified in a similar way. For example, the locus of gene OCA1 may be written "11q14-q21", meaning it is on the long arm of chromosome 11, somewhere in the range of sub-band 4 of band 1, and sub-band 1 of band 2.

The ends of a chromosome are labeled "ptel" and "qtel", and so "2qtel" refers to the telomere of the long arm of chromosome 2.