This tool is the mainstay of the modern handyman's arsenal. It consists of a largish body (though they come in a variety of sizes, from keychain to whopper) and a smaller moving jaw. A worm gear moves the moving jaw back and forth, providing a pair of parallel tool-steel surfaces at a variable distance. The idea is that you adjust the jaws until the distance apart matches the size of the nut or bolt you're dealing with, at which point the crescent wrench replaces perfectly the dedicated wrench of that exact size.

Unfortunately this idea is better in theory than in practice¹, especially if you're buying cheap imported crescent wrenches. The worm gear inevitably has some 'slop' or play in it, and the jaws tend to move apart as you use the tool, until ultimately it slips around the corners of the bolt.

This leads to the other definition of crescent wrench, used by mechanics who wouldn't have one in their toolbox: 'A tool for rounding the corners off of bolt heads.'