A door is a panel that makes an opening in a building, room or vehicle. Doors are usually made of a hard, semi-permeable, and hard-to-break substance (such as wood or metal), but sometimes consisting of a hard frame into which windows or screens have been fitted. Doors are often attached by hinges to a frame. Doors make ingress into or egress from a building, room, or vehicle easier to manage. The panel may be moved in various ways (at angles away from the frame, by sliding on a plane parallel to the frame, by folding in angles on a parallel plane, or by spinning along an axis at the center of the frame) to allow or prevent ingress or egress. In most cases, a door's interior matches its exterior side. But in other cases (e.g., a vehicle door) the two sides are radically different.
The Greek scholar Heron of Alexandria created the earliest known automatic door in the 1st century AD during the era of Roman Egypt. The first foot-sensor-activated automatic door was made in China during the reign of Emperor Yang of Sui (r. 604618), who had one installed for his royal library. The first automatic gate operators were later created in 1206 by Arab inventor Al-Jazari.
In the Renaissance period the Italian doors are quite simple, their architects trusting more to the doorways for effect; but in France and Germany the contrary is the case, the doors being elaborately carved, especially in the Louis XIV and Louis XV periods, and sometimes with architectural features such as columns and entablatures with pediment and niches, the doorway being in plain masonry. While in Italy the tendency was to give scale by increasing the number of panels, in France the contrary seems to have been the rule; and one of the great doors at Fontainebleau, which is in two leaves, is entirely carried out as if consisting of one great panel only.
A bifold door is a unit that has several sections, folding in pairs. Wood is the most common material, and doors may also be metal or glass. Bifolds are most commonly made for closets, but may also be used as units between rooms. Bi-fold doors are essentially now doors that let the outside in. They open in concert; where the panels fold up against one another and are pushed together when opened. The main door panel (often known as the traffic door) is accompanied by a stack of panels that fold very neatly against one another when opened fully, which almost look like room dividers.
French doors are derived from an original French design called the casement door. It is a door with lites where all or some panels would be in a casement door. A French door traditionally has a moulded panel at the bottom of the door. It is called a French window when used in a pair as double-leaved doors with large glass panels in each door leaf, and in which the doors may swing out (typically) as well as in.