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An adjustable wrench (US) or adjustable spanner (UK) is a wrench with a "jaw" of adjustable width, allowing it to be used with different sizes of fastener head (nut, bolt, etc.) rather than just one fastener, as with a conventional fixed spanner. An adjustable spanner may also be called a Bahco (European usage, see below), crescent wrench (US and Canada incorrect usage - see Famous brands section), adjustable end wrench (US), wrench, shifter, shifting spanner (UK), shifting adjustable, fit-all or adjustable angle-head wrench.
Forms and names 
In many European countries (e.g. France, Germany, Portugal, Spain and Italy) the adjustable wrench is called an "English key" as it was first invented in 1842 by the English engineer Richard Clyburn. Another English engineer, Edwin Beard Budding, is also credited with the invention. Improvements followed: on 22 September 1885 Enoch Harris received US patent 326868 for his spanner that permitted both the jaw width and the angle of the handles to be adjusted and locked. Other countries, like Denmark, Poland and Israel, refer to it as a "Swedish key" as its invention has been attributed to the Swedish inventor Johan Petter Johansson, who in 1891 received a patent for an improved design of the adjustable spanner that is still used today. Johansson's spanner was a further development of Clyburn's original "screw spanner". In some countries (e.g. Czech Republic, Egypt, Hungary, Iran, Slovakia, Slovenia, Poland, Romania, Bulgaria) it is called "French key" (in Poland, "Swedish" or "French" key depending on type). In the USA, a spanner is universally called a wrench.
There are many forms of adjustable spanners, from the taper locking spanners which needed a hammer to set the movable jaw to the size of the nut, to the modern screw adjusted spanner. Some adjustable spanners automatically adjust to the size of the nut. Simpler models use a serrated edge to lock the movable jaw to size, while more sophisticated versions are digital types that use sheets or feelers to set the size.
Proper use 
The movable jaw should be snugly adjusted to the nut or bolt head in order to prevent damage to the fastener's head, or rounding. In addition, it is important when applying significant force to ensure that the fixed jaw "leads" the rotation (it follows its tip) and the movable jaw "trails" the rotation, leaving its tip behind, so to speak. The area of contact for the fixed jaw should be farther from the body of the tool. That means the movable jaw's area of contact is relatively close to the body of the tool, which means less bending stress. The fixed jaw can withstand bending stress far better than can the movable jaw, because the latter is supported only by the flat surfaces on either side of the guide slot, not the full thickness of the tool.
This type of spanner should not be used on a rounded off nut, as this can overload the movable jaw. Nor should such a spanner be used "end on" in cramped quarters (except perhaps when the nut is barely more than finger-tight), where a ratchet is more appropriate.
Some cheaper brands' jaws move when twisting on tight nuts.
Famous brands 
In the United States and Canada, the adjustable spanner (adjustable wrench) is colloquially referred to as a "crescent wrench" due to the widespread Crescent brand of adjustable wrenches. The Crescent brand of hand tools is owned and marketed by Apex Tool Group, LLC. In some parts of Europe, adjustable spanners are often called a Bahco. This term refers to the company of the Swedish inventor Johan Petter Johansson, which was originally called B.A. (Bernt August) Hjort & Company. The Swedes themselves ironically call the key "skiftnyckel" which is translated into adjustable key (shifting key).
See also 
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