Autosomal or somatic chromosomes (ie not sex chromosomes) are the chromosomes that are identical in male and female members of the same species. They are numbered in ascending order:

chromosome 21 [lower-case c; arabic numerals]

Allosomes (sex chromosomes) use letters instead of numbers. There are 2 main sex-determination systems: XY (humans, most other mammals, some insects, some plants) and ZW (birds; some fish, crustaceans, insects and reptiles). These are generally written as:

X chromosome [capital X and lower-case c]     Z chromosome