 Most Edison screws have right-hand threads (lamp is turned clockwise to tighten), but left-hand threaded screws are sometimes used, usually for a non-standard voltage or wattage bulb.This prevents the use of an incorrect bulb, which could cause damage.Public locations such as railway trains and the New York City Subway have used light bulbs with left-hand threads to discourage theft of the bulbs for use in regular light fixtures.