It usually consists of a pair of plates, each with a set of open cylindrical rings (the knuckles) attached to them.
Doors generally have at least one fixed handle, usually accompanied with a latch (see below).
On some doors the latch is incorporated into a hinged handle that releases when pulled on.
Functionally, all but swinging doors use some form of fastener to hold them closed.
Typical forms of fasteners include: Numerous devices exist to serve specific purposes related to how a door should (or should not) be used.