Number of Adenine residues are always equal to the number of thymine residues.
Number of guanine residues are always equal to number of cytosine residues.
Number of purines (A and G) combined and the number of pyrimidines (T and C) combined are always equal to each other (obvious consequence of first 2 rules!)
Chargaff had also determined that the same findings hold true even if the ratio of pyrimidines and purines may differ from one kind of living organism to another.
When combined with the findings of Rosalind Franklin about the diffraction studies on DNA, it was also determined how base-paring between A and T as well as G and C is what is behind the double -helix structure of the DNA and that no other combinations of these residues are possible.
He explained the same findings to Francis Crick and James Watson who were then the ones who enlightened the world about the double-helix DNA structure.