The role of second-order perturbations in interlayer interactions of two-dimensional electronic systems is examined. The general features of interlayer interactions are deduced from a simple two-layer model. The preferred motion is discussed specifically for the stacking of layers in graphite, using a transition-density analysis for extended systems. The well-known ABAB stacking of layers in graphite is proposed to be a consequence of orbital interactions between layers rather than of generally accepted van der Waals forces.