Isothermic parameterizations are synonyms of isothermal curvature line parameterizations, for surfaces immersed in Euclidean spaces. This short article presents a conjugation relationship between the mean curvature and the Hopf differential which correspond to a pair of dual isothermic surfaces, f and  respectively.

This relationship is natural, considering that integrable surfaces are defined as solutions of integrable systems (Lax systems based on moving frames). For any given Riemannian metric, every integrable surface is born from a couple of parents: the mean curvature H and the Hopf function Q. For any two isothermic surfaces that are dual to one another, the couple of parents is essentially the same, but the mother and father reverse roles, which leads to a conjugation formula for  and  that is proven in an elementary way.

isothermic coordinates, isothermic surfaces, duality, mean curvature, Hopf differential.