door at floor level, since the cleanest air is at floor level and one may be crawling to get to the exit, thus placing the key where it is easiest to find. Note that single cylinder dead locks (with an unlocked twist mechanism on the inside of the door) do not have this problem, and therefore are most commonly used on fire exits. Some areas have fire safety codes that do not allow a locked exit. See also Door security Lock bumping References Doors Locksmithing Locks (security device)