A+T and G+C base pairs have similar structures
The combination of a two-ring purine (A or G) and a single-ring pyrimidine
(T or C) forms a base bair (A+T or G+C ) with three
co-planar rings. The distance between the two deoxyribose
sugars in either strand is essentially identical for
The A+T has two H-bonds versus the G+C with three.
It may be convenient to think of an "A" as having two toes that attach to the two "ears" of the "T".