The unusual manner of entropy change at the ice Ih<->ice XI transformation has been interpreted in terms of an equilibrium involving the strain energy at the ice Ih/ice XI interface, the Gibbs energy and the grain-boundary energy, and a formalism given. The persistence of;Ih below its first order phase transformation temperature of 72 K and the persistence of ice XI, the proton-ordered phase, at temperatures up to 110 K are attributed to the predominance of the strain energy which opposes the transformation favored by the Gibbs energy. Calculations using the measured entropy yield a strain energy of 3.5 J/mol at 72 K, which decreases on heating toward 120 K. Consequences of the theory are : (i) Thermodynamic quantities measured below 72 K would change on thermally cycling the sample through the transformation region, which has been observed; (ii) A procedure that reduces the strain at the ice nh-ice XI boundary will increase the extent of transformation, i.e., mechanically deforming or grinding a partially transformed sample, or subjecting it to high energy irradiation to create more point defects that can bind with the ice Ih/ice XI interface, would transform ice Ih and ice XI more in either direction.