A ratchet spanner or wrench uses a mechanism which allows an open-ended, flare or ring head to turn a fastener in one direction but not the other while remaining fitted to the fastener head. By turning the spanner over or flicking a lever, it changes the direction of movement. They are an essential tool in any toolbox as it can tighten and loosen a nut or bolt, and by applying the correct amount of torque to the fastening, it can save time and effort. Some manufacturers choose to etch the arrows of the torque direction near the lever, making it easier for the user to identify the direction to tighten or loosen in. Ratchet spanners are available in a variety of metric and imperial sizes