Although the term "crescent wrench" is thrown around as though it were a type of tool, crescent wrenches got their name from the Crescent Tool Company in the early 1900s. It was first created by inventor Karl Peterson, although a wrench by the Swedish company BAHCO may have inspired Peterson's design. The term crescent wrench has since become a generic name for a type of adjustable wrench. Crescent wrenches have one fixed jaw and one movable jaw; they differ from monkey wrenches in that the jaws on a crescent wrench are nearly parallel to the handle, whereas the jaws on a monkey wrench are perpendicular to the handle. In both types, the jaws are adjusted using a worm gear that you turn with your thumb.
Crescent-brand wrenches come in all forms nowadays, but there were four basic historic types of Crescent wrench: the 8-to-10-inch double-ended adjustable wrench; the 6-to-8-inch double-ended adjustable wrench; the 8-inch adjustable wrench; and the 12-inch adjustable wrench. They differed in size, jaw opening and whether they had one set of jaws on one end, or a set on each end.
Over time, adjustable wrenches acquired the nickname of "knucklebuster" because of the tendency of the jaws to slip off whatever you're loosening, causing you to scrape your hand against the nearest surface. That's why it's important when you're using an adjustable wrench that you position the wrench so that you're pulling toward yourself and not a potentially dangerous knuckle-busting surface. Other important adjustable-wrench rules are that the jaws should tightly hug at least three points on the nut or bolt you're loosening so you have a good grip; that you retighten the jaws any time you move the wrench to a new position; and that you position the sliding jaw so that it faces you in order to put the most pressure on the fixed jaw.