Cellular functions are mediated by protein-protein interactions. How proteins bind interaction partners is largely an open problem. The state of our understanding in protein-protein interactions could benefit greatly from a simple model to probe the complexities of the subject. The knob-socket model has been shown to be an important motif for protein tertiary structure packing. The knob-socket is a four residue tetrahedral motif calculated using Voronoi Polyhedra, where the knob from one secondary structure element packs into a socket formed by three residues from another secondary structure element. It has been shown that the knob-socket model naturally leads to an amino acid code for protein structure. The work presented extends the knob-socket analysis of protein tertiary structure to quaternary structure packing. Using the 3D Complex database of protein-protein interactions, the amino acid composition of knob-sockets at the interface between proteins is described. The extent to which protein tertiary and quaternary structure are similar is described in terms of the knob-socket construct. Furthermore, this work illustrates how knob-socket data can design helices that homodimerize in solution. In conclusion, knob-socket approaches to protein-protein interactions provide a clear and concise classification of protein-protein interactions into three separate classes. Class 1 is defined through peptide-protein interactions. Here, the peptide provides knobs that pack into the already established free sockets of the protein interaction surface. Class 2 is characterized by knobs-sockets shared between the two protein interaction partners. Class 3 is characterized by structure formation upon interaction. Class 3 exhibits induction of protein structure with knob-socket packing from both chains. This can occur for both protein-protein interactions as well as peptide-protein interactions. This knob-socket analysis provides unique insights into the complexity of protein-protein interactions.