Einstein, Theodore L.Jung, T. M.Bartelt, N. C.Williams, Ellen D.Rottman, CraigThis article appeared in the Journal of Vacuum Science & Technology A and be found at https://doi.org/10.1116/1.577944.We discuss two types of step?height doubling transitions on vicinal surfaces. In one type, exemplified by Ge(111)(121), a phase transition can occur since the symmetry between alternate terraces is broken at low temperatures. There is evidence in both experiment and in Monte Carlo simulations that this transition can have Ising character. In the second type, exemplified by Si(001)(110), one subset of terraces is always favored, so that no symmetry is broken. If an actual phase transition occurs (rather than a smooth crossover), it is expected to be first order, implying a coexistence region in a temperature?misorientation phase diagram. Finally, the coalescence of steps into steps of height more than two atomic layers is briefly considered.Article