Th e bases, both purines and pyrimidines, are fl at and planar, so they can stack on top of one another. In addition, the atoms of the aromatic rings are very polariz- able, and many of the atoms have a partial charge. Th ese two factors make the combination of van der Waals and electrostatic interactions between stacked bases particularly strong. Th ese eff ects are known collectively as base-stacking interactions, and they provide the dominant contribution to stabilizing the dou- ble-helical conformation of nucleic acids.