A hinge is a component that attaches one edge of a door to the frame, while allowing the other edge to swing from it.
It usually consists of a pair of plates, each with a set of open cylindrical rings (the knuckles) attached to them.
The knuckles of the two plates are offset from each other and mesh together.
A hinge pin is then placed through the two sets of knuckles and usually fixed,
to combine the plates and make the hinge a single unit.
One door usually has about three hinges, but it can vary.