sentences:  They can open up and fold either internally or externally and are widely used in modern extensions.Opposed to a conventional door with a standard aperture of 700-900 mm, most modern folding door (sets) can be up to several meters in width, and thereby considerably increase both physical and visual accessibility between either two internal rooms or an internal room and an outside space.Historically folding doors were already known by the Romans as excavations in Pompeii have revealed.