The markings on each of the chromosomes are very important in identifying which chromosome is which.  If you look closely at the karyotype below, you will see that there are subtle differences between the chromosomes.

Each chromosome has a characteristic black and white banding pattern and has a constriction (called a centromere) in a characteristic location somewhere along its length. These two things, the banding pattern and the centromere, make each chromosome recognizable to a trained eye.

As you can see, the centromere of each chromosome is not exactly in the middle of the chromosome.  This makes the chromosome appear as if it had two distinct segments of unequal length. These segments are called arms.  The shorter arm (called "p" for petit) is always shown on top.  The longer arm is called the “q” arm and is shown below the p arm.

The bands on each chromosome arm divide it up into regions. The regions are numbered starting at the centromere and progressing to the end of the chromosome arm.   Below is a diagram that shows how the bands of chromosome 18 are labeled.

^{_{Idiogram Album: Human copyright 1991 David Adler and Michael Willis}}