A rule followed by the nucleotides isolated from living cells. Erwin Chargaff, during his studies of nucleic acids, discovered that no matter what the biological source of the DNA, the ratio of the nucleotides obeyed this simple formula:
% Adenine = % Thymine
% Guanine = % Cytosine
Chargaff didn't think this was of great importance (at least in the movie The Race for the Double Helix), but as it turned out, it was one of the key pieces of data which allowed James Watson and Francis Crick to come up with the double-helix structure of DNA. The ratio arises because in normal cells, nearly all of the DNA is present as a complimentary double-helix, meaning that for each Adenine there is a corresponding Thymine, for each Guanine there is a corresponding Cytosine, and vice versa.
Erwin Chargaff was no fool. He just missed the importance of one of his findings. He made many other contributions to the study of nucleic acids.