A theory of the statistical mechanics of the phase transition between an ordered alpha helix state of a polypeptide and the disordered random state of the polypeptide caused by a change in temperature or some other change in physical conditions. This model was proposed by Bruno Zimm and J. K. Bragg in 1959. It is related to the one-dimensional Ising model and can be solved exactly. Refinements and extensions of the model have been proposed.