Chargaff rule: The rule that in DNA there is always equality in quantity between the bases A and T and between the bases G and C. (A is adenine, T is thymine, G is guanine, and C is cytosine.) Named for the great Austrian-American biochemist Erwin Chargaff (1905-2002) at Columbia University who discovered this rule. Also known as Chargaff's ratios. Chargaff later said: "This observation of complementarity, later called Chargaff's ratios, was essential to the solution of DNA's structure. In hindsight, the complementary pairing of the nucleotides powerfully suggested that a DNA molecule could break into two parts. Only complementary bases could form bonds and line up in place in a new DNA strand."