A hinge converts the swinging motion of a door into rotation around a fixed vertical pin.

The longer version

Two plates joined by a pin let a door swing while staying attached. One plate is fixed to the frame and the other to the door, and the pin running through their interlocking knuckles defines the axis. Every point on the door travels in a circle around that pin, so a heavy door moves without lifting or dragging.

Alignment of the pin is what makes the swing feel right: if the axis is not vertical the door drifts open or shut on its own. Load passes downward along the pin and into the frame, which is why screws loosening at the top hinge is such a common fault. A third hinge simply shares that load over more fixings.

Where this entry sits

Odd Little Things looks at the ordinary objects people touch every day and the small mechanisms and histories hiding inside them. Buttons, zips, pencils, mirrors, stairs, stamps, packaging, and road signs all get their turn. The full list sits on the Odd Little Things page, where every entry is listed in order.