An adjustable spanner, shifting spanner, shifter, crescent wrench or adjustable-angle head wrench (American English) is a tool which can be used to loosen or tighten a nut or bolt. It has a "jaw" (the part into which the nut or bolt goes) which is of adjustable size, which allows for different size nuts and bolts to be handled by the same spanner. Compare this to the ordinary spanner which has a fixed size.
There are many forms of adjustable spanners, from the taper locking spanners which needed a hammer to set the moveable jaw to the size of the nut, to the modern screw adjusted spanner.
There is a class of adjustables that automatically adjust to the size of the nut. The most modern are digital types that use sheets or feelers to set the size, and other simpler models that use a serrated edge to lock the moveable jaw to size.
It is uncertain who invented the adjustable spanner. Some early spanners were invented by Edwin Beard Budding (1795–1846) using a screw to replace the wedge that fixed the jaw of a known type of adjustable spanner, and Johan Petter Johansson of Sweden in 1892 using a screw to adjust and fix the jaw. Monkey wrenches are another type of adjustable wrench with a long history; the origin of the name is unclear.
The movable jaw should be snugly adjusted to the nut or bolt head in order to prevent rounding. In addition it's important to ensure that the movable jaw is located on the side towards which the rotation is to be performed. This reduces the risk of deformation of the movable jaw or the adjusting mechanism, thus avoiding the increase of backlash.
This type of wrench should never be used on a rounded off nut, as this can overload the movable jaw. Nor should the wrench be used "end on" in cramped quarters, where a socket wrench is more appropriate.