Chromosomes can be distinguished according to certain criteria in a karyogram.
Size of chromosome und position of centromere: Chromosomes are sorted in categories A to G based on the position of their centromere, which divides the chromosome into a p-arm and a q-arm, and their size.
Banding pattern: Chromosomes have several bands, which can be visualised with a fluorescenct dye or by Giemsa-labelling. Thereby, 300-500 bands can be distinguished, which help to further sort chromosomes into sub-groups.
Source: Bsteinmann, karyogram by Dr. Klaus R. Sandig.