A structural tree for α-helical proteins and domains including α-α-corners has been constructed. The α-α-corner is taken as a root structure of the tree. The larger protein structures are obtained by stepwise addition of a-helices to the root α-α-corner taking into account a restricted set of rules inferred from known principles of protein structure. The protein structures that can be obtained in this way are grouped into one structural class and those found in branches of the tree into subclasses.
