Conventional screws have right-handed threads. To tighten a screw, turn the head or bolt clockwise, moving its top to the right. To loosen, turn counterclockwise, moving the top to the left. The famous physicist Hans Bethe was reputed to remember the convention by relating it to the right-hand rule for vector products:
Some flammable gas couplings use left-handed threads, to prevent dangerous misconnections.