Ideogram

Geneticists use diagrams called ideograms as a standard representation for chromosomes. Ideograms show a chromosome's relative size and its banding pattern. A banding pattern is the characteristic pattern of dark and light bands that appears when a chromosome is stained with a chemical solution and then viewed under a microscope. These bands are used to describe the location of genes on each chromosome.

Ex: If a chromosomes is has Giemsa banding pattern

The parts of the chromosome having a positive reaction to the staining will be dark (G bands). R (reverse) bands have a negative reaction to the staining and are light.

Heterochromatin stains dark and usually has no actively transcribed genes
Euchromatin stains light and may or may not have actively transcribed genes