Structural trees for large protein superfamilies, such as b proteins with the aligned b sheet packing, b proteins with the orthogonal packing of a helices, two-layer and three-layer a/b proteins, have been constructed. The structural motifs having unique overall folds and a unique handedness are taken as root structures of the trees. The larger protein structures of each superfamily are obtained by a stepwise addition of a helices and/or b strands to the corresponding root motif, taking into account a restricted set of rules inferred from known principles of the protein structure. Among these rules, prohibition of crossing connections, attention to handedness and compactness, and a requirement for a helices to be packed in a-helical layers and b strands in b layers are the most important. Proteins and domains whose structures can be obtained by stepwise addition of a helices and/or b strands to the same root motif can be grouped into one structural class or a superfamily. Proteins and domains found within branches of a structural tree can be grouped into subclasses or subfamilies. Levels of structural similarity between different proteins can easily be observed by visual inspection. Within one branch, protein structures having a higher position in the tree include the structures located lower. Proteins and domains of different branches have the structure located in the branching point as the common fold.