Most locks are closed by chevron or mitre gates rather than by caissons. These are pairs of hinged gates that form a "V" shape, with the deeper water outside the V. Water pressure thus holds them closed. These gates can be opened and closed quickly, so they are used for canal locks, to change levels, and also for most freight docks. As the function of a freight dock is to enclose deep water, such gates point inwards. A graving or dry dock, in contrast, excludes water, and so their gates point outwards.
