Arguably, French doors could be considered as retractable as each door can be pulled back. The advantage of retractable doors is that maximum access is possible between the two sides of the aperture whereas sliding panel doors generally overlap unless built to retract into a recess.
Because slide-and-pivot doors have no hinges, there is no requirement for a sturdy side frame; its only purpose is to cover the gasket that seals the double glazed unit. This means that the views afforded through the expanse of patio doors have minimal interruptions. At the time of writing, there are two versions of frame-less glass doors available in the UK, both using the slide-and-pivot technique: one manufacturer supplies their frameless glass doors with kite-marked double glazed units which have a visible seal, the other uses an almost transparent method of sealing their double glazing. Contemporary by design, the absence of visible characteristics makes frameless glass doors a viable option for period properties.
Patio doors comprise an outer frame plus individual door frames. These can be made from wood (soft- and/or hard-wood), metal or alloys (usually aluminium), pvc (polyvinyl chloride, a thermo-plastic polymer - the u stands for unplasticised) or a composite material, which may comprise any of the foregoing materials plus grp (glass reinforced polymer). There is also a style known as frameless, where the vertical sides of each door have no frames.
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