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A helix-coil transition model assigns local states of a biopolymer to an ordered helical conformation or a disordered coil conformation. Their competing free energies determine the tendency to form a helix, while the cost of boundaries between regions gives cooperativity: forming a new helical run can be less favorable than extending an existing one. A coil state is a class of disordered conformations, not a claim that the whole molecule is an ideal chain. The Zimm-Bragg model is a simple nearest-neighbour statistical model of this competition.