Chargaff's rules is a two main rules of nucleotide distribution in DNA strings, discovered by Austrian chemist Erwin Chargaff in early 1950s in Columbia University.
First Chargaff's rule (or first parity rule) holds that in double-stranded DNA molecule observed percentage base pair equality: %A = %T and %G = %C. This finding, with the results of x-ray diffraction analysis by Rosalind Franklin, served as one of the grounds for the Watson-Crick double helix model.
Second Chargaff's rule (second parity rule) holds that for each of the DNA strands observed following approximate equality: %A ~ %T and %G ~ %C. It was an empirical observation, and the basis for this rule is still under investigation. It was shown that it does not apply to organellar genomes (mitochondria and plastids) smaller than ~20-30 kbp, single stranded DNA (viral) genomes or any type of RNA genome.