Spanner. The width across flats indicates the nominal "size" of the spanner. It is imprinted on the spanners in millimeter (mm) values. Older British and current US spanners (wrenches) have inch sizes that are imprinted in intermediate sizes in fractions.

There are many types of spanner wrenches, each with a distinctly shaped hook. In addition, some spanners are multi-tools with double-sided hooks. The most common type of spanner wrench is the C spanner. Its head is open in the shape of the letter C, and its jaws are meant to correspond to the similarly sized nut.

The length of an adjustable wrench refers to the distance from the handle's end to the jaw's end when the wrench is fully extended. Adjustable wrenches are available in different lengths, typically 4 to 18 inches or more. Longer wrenches offer increased leverage, making them suitable for higher-torque applications.