Complementarity is the key to understand the stability of the double helix of DNA in the Watson-Crick model (the most stable among other conformations). DNA complementarity property is determined by the rules for base pairing between A-Tand G-C. In a "perfect duplex DNA" (or Watson-Crick model), the strands are precisely complementary. If we compare two different but related double-stranded molecules, therefore, each strand of the first molecule will be similar to one strand of the second one and will be (partly) complementary to the other strand of the second molecule. This property of the molecule of DNA was the key to elucidate the correct correspondence between strands and to propose the model in that historical paper by James Watson and Francis Crick in 1953 [Nature, 171, 737 (1953)], were they wrote:"If it is assumed that the bases only occur in the structure in the most plausible tautomeric forms(that is, with the keto rather than the enol configurations) it is found that only specific pairs of bases can bond together. These pairs are adenine (purine) with thymine (pyrimidine) and guanine (purine) with cytosine (pyrimidine).In other words, if an adenine forms one member of a pair, on either chain, then on these assumptions the other member must be thymine; similarly for guanine and cytosine. The sequence of bases on a single chain does not appear to be restricted in any way. However, if only specific pairs of bases can be formed, it follows that if the sequence of bases on one chain is given, then the sequence on the other chain is automatically determined.It has been found experimentally that the ratio of the amounts of adenine to thymine, and the ratio of guanine to cytosine, are always very close to unity for deoxyribose nucleic acid." (In perfect agreement with the Chargaff's rules)