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 sugar components of the backbones of either strand is essentially identical for either pair.

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 are paired to the two "ears" of the "T".

Figure © 2012 TA Brown, Introduction to Genetics (1st ed.); additional text © 2014 by Steven M. Carr