What rule from Erwin Chargaff did Watson and Crick use?

The rule from Erwin Chargaff that Watson and Crick used was Chargaff's rules. Chargaff's measurements showed that DNA contains approximately equal amounts of adenine and thymine, and equal amounts of guanine and cytosine.

These ratios suggested that the bases were not arranged randomly. Watson and Crick realized that adenine could pair with thymine and guanine with cytosine, producing pairs of similar overall dimensions inside a regular double helix. This was a crucial clue alongside X-ray diffraction evidence.

Chargaff developed his rules from DNA samples taken from different organisms in the late 1940s and early 1950s. The rules are often simplified as A = T and G = C, but they do not mean that all four bases occur in equal amounts. The overall GC content can vary among species, and the rules apply most directly to double-stranded DNA.