he door has been a basic part of every building since people lived in caves. There are many different variations of doors, but they fall into two main classifications.The main way that the different variations of doors are classified is between interior and exterior doors. The rather simple classification is actually more important than just the physical location of the door. This is because the functions of the doors differ greatly and so construction and hanging methods often are different on exterior or interior doors.The exterior door is an opening from the building to the outside. It is often designed to provide security and protection from hostile elements. Obviously, the construction methods of an exterior door will make it thicker and stronger to serve these two purposes. The exterior door will be hung in a way that makes it stronger and less able to be forced by an intruder. Many exterior doors will have openings such as the Dutch door that combine the purpose of a door and a window.

Interior doors provide access between rooms inside the building. In private homes, the interior doors are usually hollow wooden doors. They are not needed to provide the same type of security and protection from elements as the exterior doors. They serve more for privacy. In some commercial buildings, a door might still need to provide a security purpose and can be made to the same standards and of the same materials as an exterior door.

Different variations of doors can be classified by the means of opening. Although most doors are hinged, they can open inward or outward. Some doors are made to fold and others, like patio doors, are tracked and slide open and shut. Of course, material can be used as another way of classifying the various different variations of doors. Sliding glass doors, heavy metallic exterior doors, and lightweight folding closet doors are examples.

Doors, like windows, serve a functional purpose in a building. This does not mean that they also do not serve a decorative purpose as well. Although the appearance and material of a door is dictated mostly by its location and function, there is a lot of leeway in imaginative decorative ideas that can integrate the door into the overall design. In other words, a door does not have to be dull.