When open, doors admit people, animals, ventilation or light. The door is used to control the physical atmosphere within a space by enclosing the air drafts, so that interiors may be more effectively heated or cooled. When closed, a door normally impedes the transfer of air from one side to the other. Similar structures that do allow air to be transferred through some form of a grillwork are called gates.
Doors may have an aesthetic purpose in creating an impression of what lies beyond; for example, keeping administrative and factory areas of a building separate. In less formal settings, doors may also be seen as a sign of the desire for privacy. As a form of courtesy and civility, people often knock before opening a door and entering a room.
Doors are often symbolically endowed with ritualistic purposes. For example, being granted access to a door, including the guarding or receiving of the keyto that door, may have special significance. Similarly, doors and doorways frequently appear in literature and the arts in metaphorical or allegoricalcontext, often as a portent of change.
In most cases the interior side of a door matches its exterior side but, in some other cases, there are sharp contrasts between the two sides, such as in the case of a vehicle door.