Some chromosomes are much longer than others. Second, the position of the centromere relative to the end of the chromosome is used to help tell chromosomes apart. Centromeres are designated either metacentric, submetacentric, or acrocentric. Metacentric chromosomes have their centromere near the center of the chromosome. Submetacentric chromosomes have slightly off-center centromeres, such that one chromosome arm is longer than the other.
Acrocentric chromosomes have the centromere located very near to one end.
A third way is to stain them with chemical dyes. Staining results in patterns of bands that are characteristic for each chromosome pair. 46 is the normal number of chromosomes. The two XX's indicate the normal composition of sex chromosomes for a female. When chromosomal material is exchanged between different chromosomes, it is called a translocation.