Textile membrane roofs span great distances, are light-weight and translucent, vibrant and adaptable, and are even used as retractable roofs. They combine load-bearing and protective functions in one. Masts and guying cables define the high and low points, thus producing the required surface curvature. Because of the interdependence of external shape and inner forces, membrane structures cannot be designed at will. Instead, their form is the result of a search for, and determination of, an equilibrium.