2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/152029We 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 Möbius invariant frame for an isothermic surface is built.9 pages, AmSLaTeX 1.1, no figuresDifferential Geometrytext