A socket wrench is a type of wrench, or tightening tool, that uses separate, removable sockets to fit many ifferent sizes of fittings and fasteners, most commonly nuts and bolts. It generally includes a ratcheting mechanism that allows the nut to be tightened or loosened with a continuous motion, rather than requiring that the wrench be removed and refitted after each turn. Typically, a lever behind the socket switches the wrench between tightening and loosening modes. The sockets attach to the ratchet through a square fitting that locks onto them and come in four common sizes: 1/4 inch, 3/8 inch, 1/2 inch, and 3/4 inch. These different square measures are described as "drives." (Despite being denominated in inches, these are international standards and no "metric" counterparts exist.) Larger drive sizes such as 3/4 inch, 1 inch, and above are usually reserved for use on fasteners used on larger industrial vehicles, in shipyards, and the like. The sockets themselves come in a full range of inch (SAE) and metric sizes. The two should not be substituted for each other, even if the fit seems "close" - it may destroy the nut or bolt head. The sole exception is the 3/4" and 19mm sockets, which vary by only one twentieth of one millimeter (less than the width of a human hair). The advantage of the socket wrench system is speed - it is much faster than a conventional wrench, especially in repetitive bolt-on or bolt-off usage.