Watson and Crick based their model on Franklin's X-ray diffraction photograph.  Their model had a uniform width of 2 nanometers and bases stacked .34 nanometers apart.  The width suggested the presence of 2 strands (as Linus Pauling thought) not 3 (as Wilkins initially proposed).

Franklin's X-ray data implied the helix makes a full turn every 3.4 nanometers; because .34 nanometers apart, there are 10 layers in each full turn.  At the time Adenine and Guanine were known to be the same type:  purines; as well as Thymine and Cytosine:  pyrimidines

• purine + purine = too wide; pyrimidine + pyrimidine = too narrow; pyrimidine + purine = consistent with X-rays
            Why?
                      Adenine and Thymine have 2 binding sites (hydrogen bonds).
                      Cytosine and Guanine have 3 binding sites (hydrogen bonds).
                      Any other arrangement would make DNA unstable.
                      This complies with Chargaff's Rule.

Next:  "Meselson, Stahl and DNA replication."