A description of elements of protein structure can be viewed as a segmentation of structure space into regions of recurrent motifs.
Proteins are generally much too large to be easily comprehended.
Classifying patterns of structure in proteins is one of the most fruitful approaches to the organization of such complex systems. The confusing jumble of atomic coordinates and bonds may be parsed into its constituent patterns of structure, or motifs, analyzed as to how these are connected in specific folds, how these are organized in structural and functional subunits, or domains and in what relationship these can be placed with other proteins in structural or functional families. Historically, the first protein structures were perceived to be unique and confusing. On the other hand, it is by far not the only way to bind nucleotides. Key to the question of regularity in protein structure is the concept of structural similarity.