A door is a moving structure used to block off, and allow access to, an entrance to or within an enclosed space, such as a building or vehicle. Doors normally consist of a panel that swings on hinges on the edge, but there are also doors that slide or spin inside of a space. Similar exterior structures to doors are called gates.
Typically, doors have an interior side that faces the inside of a space and an exterior side that faces the outside of that space. In many cases the interior side of a door mostly matches its exterior side, but in some other cases there are sharp contrasts between the two sides, such as in the case of the vehicle door.
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. Doors are significant in preventing the spread of fire. They also act as a barrier to noise. Many doors are equipped with locking mechanisms to allow entrance to certain people and keep out others. As a form of courtesy and civility, people often knock before opening a door and entering a room.