With the use of a recently developed method, twenty-four proteins for which two or more X-ray conformers are known have been analyzed to reveal structural principles that govern domain motions in proteins. In all 24 cases, the domain motion is a rotation about a physical axis created through local interactions both covalent and noncovalent. In many cases, two or more mechanical hinges separated in space create a stable hinge axis for precise control of the domain closure. The terminal regions of β£-helices and β€-sheets have been found to act as mechanical hinges in a significant number of cases. In some cases, the two terminal regions of neighboring strands of a single β€-sheet can create a hinge axis, as can the two termini of a single β£-helix. These two structures have been termed the ''double-hinged β€-sheet'' and ''doublehinged β£-helix,'' respectively. A flexible loop that attaches one domain to another and through which the effective hinge axis passes is another construct that is used to create a hinge. Noncovalent interactions between segments remote along the polypeptide chain can also form hinges. In addition β£-helices that preserve their hydrogen bonding structure when bent have been found to behave as mechanical hinges. It is suggested that these β£-helices act as a store of elastic energy that drives the closing of domains for rapid capture of the substrate. If the repertoire of possible interdomain structures is as limited as this study suggests, the dynamic behavior of proteins could soon be predicted using bioinformatics techniques.