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 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 attach to the two "ears" of the "T".

Figure 2012 TA Brown, Introduction to Genetics (1st ed.); additional text 2013 by Steven M. Carr