A folding door is a type of

door A door is a hinged or otherwise movable barrier that allows ingress (entry) into and egress (exit) from an enclosure. The created opening in the wall is a ''doorway'' or ''portal''. A door's essential and primary purpose is to provide security b ...

which opens by folding back in sections or so-called panels. Folding doors are also known as ‘bi-fold doors', in spite of them most often having more than two panels. Another term is ‘

concertina A concertina is a free-reed musical instrument, like the various accordions and the harmonica. It consists of expanding and contracting bellows, with buttons (or keys) usually on both ends, unlike accordion buttons, which are on the front. The ...

’ doors, inspired by the musical instrument of the same name. Folding doors can be used as internal or external room dividers and are made from a variety of materials. Most folding doors are glazed and the panels have frames of either

wood Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin th ...

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...

or upvc. 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 In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane. An opt ...

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 Roman or Romans most often refers to: *Rome, the capital city of Italy * Ancient Rome, Roman civilization from 8th century BC to 5th century AD *Roman people, the people of ancient Rome *''Epistle to the Romans'', shortened to ''Romans'', a lette ...

excavations In archaeology, excavation is the exposure, processing and recording of archaeological remains. An excavation site or "dig" is the area being studied. These locations range from one to several areas at a time during a project and can be condu ...

Pompeii Pompeii (, ) was an ancient city located in what is now the ''comune'' of Pompei near Naples in the Campania region of Italy. Pompeii, along with Herculaneum and many villas in the surrounding area (e.g. at Boscoreale, Stabiae), was buried ...

have revealed.

Peter Connolly Peter William Connolly FSA (8 May 1935 – 2 May 2012) was a British scholar of the ancient world, Greek and Roman military equipment historian, reconstructional archaeologist and illustrator. He was a regular contributor to such periodicals as ...

writes in his book ‘Pompeii’ that the Romans ‘disliked single-leaf doors and wherever possible, both inside and out, they used double or even folding doors’.

See also