Curved Flats and Isothermic Surfaces
  We show how pairs of isothermic surfaces are given by curved flats in a pseudo Riemannian symmetric space and vice versa. Calapso's fourth order partial differential equation is derived and, using a solution of this equation, a M\"obius invariant frame for an isothermic surface is built. 
F. Burstall, U. Hertrich-Jeromin, F. Pedit and U. Pinkall