chamber; when closed, the gate is watertight.
* A set of lock gear to empty or fill the chamber as required. This is usually a simple valve (traditionally, a flat panel (paddle) lifted by manually winding a rack and pinion mechanism) which allows water to drain into or out of the chamber. Larger locks may use pumps.

The principle of operating a lock is simple. For a boat travelling downstream, the process is:
* If the water in the chamber is low, the boat waits while the chamber is filled by the upstream valve.
* The upstream