Given the accepted tetranucleotide hypothesis, most workers assumed that deviations from equimolar base ratios were due to experimental error. Erwin Chargaff documented that base compositions actually varied among species, that in all species [ A ] = [ T ] and [ C ] = [ G ], and that the ratio [C+G] / [ A+T ] was typically less than unity (that is, [C+G] is less abundant). These ratios are referred to as "Chargall's Rules".
The molar equivalences of A vs T and C vs G intuitively suggest some sort of pairing relationship. Chargaff himself refused to speculate on the implications of his emprical observations, and regarded Watson & Crick's model building as a scientifically unsound approach. Several generations of genetics students have said to themselves, "Two strands or three, bases inside or outside, and you have Chargaff's Rules - how long can it take?"