Open jaw spanner wrenches ordinarily are cast or forged to fit one size of nut. The Crescent wrench is an adjustable design, with jaws that open from zero to the maximum jaw opening.
As described in the original patent by Hsieh Chi-Ching of Fong Yuan City, Taiwan, the Crescent wrench's main part is the handle with incorporated fixed jaw. Within this main section is also an aperture for a worm screw and a locking lever to fix the adjustable jaw in position.
Most Crescent wrenches sold today are simpler than the original patented concept. The locking lever that was to hold the movable jaw in place is no longer a part of the tool.
Although a crescent wrench is adjustable over a wide range, the physical size of the tool is limiting. Small versions lack the leverage needed for larger nuts, and large versions may not fit the available space in which to work.
Without the locking lever intended by Hsieh, the setting of the movable jaw will wander. Constant readjustment is necessary or parts being worked upon may be damaged by the poor fit.