A side view of DNA. Base pairs are stacked nearly one on top of another in the double helix. The stacked bases interact with van der Waals forces. Such stacking forces help stabilize the double helix.
[Source: J. L. Tymoczko, J. Berg, and L. Stryer, Biochemistry: A Short Course, 2nd ed. (W. H. Freeman and Company, 2013), Fig. 33.13.].