Open-ended spanners can have one head or two heads (one at either end of the shaft). Two-headed spanners will have two different sizes, either one to two size differences (e.g. 16mm and 17mm) or two common sizes (e.g. 13mm and 16mm).
Sizes of open-ended spanners range from 4mm to 50mm or 5/32″ to 2″ and the length of the tool ranges from 100mm (4″) to half a metre long (19½”). The tool gets longer as the head size gets bigger to provide enough leverage to turn the larger fasteners.
Heads of open-ended spanners are usually at a 15 degree angle to the shaft. This allows an increase in movement, creating a larger turning arc in enclosed places when the spanner is flipped over.