In all periods of human history, man has sought to create buildings with flexible outer skins – from the simple nomadic tent to the sail-like roofs used in antiquity to provide shading in theatres and stadiums. The basic type of sail roof, the velum or membrane, consisted of a wood stave construction on which the sails were gathered in or extended in a parallel movement. In modern times, advances in engineering and the development of high-strength fabrics have opened up new dimensions for movable forms of construction. Retractable roof structures can be divided into two basic types: soft forms of construction, in which the geometry can be changed by folding, gathering or rolling up the covering; and rigid forms that are moved by sliding, folding or rotating roof segments of fixed shape. Christoph Gengnagel, Rainer Barthel