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In the guitar and similar instruments, the action is the distance between the fingerboard and the string, which determines how easy it is to sound notes when pressure is applied with the fingertips. Generally a low action is considered to be more playable, due to the lower amount of pressure needed to press the string to the fingerboard. However, if the action is set too low, there is a danger that the vibrating string will strike the frets or fingerboard below it, creating an unwanted buzzing noise (on fretted instruments, this is known as fret buzz). Conversely, if the action is too high, then the strings may be too taut to fully depress.
Adjusting the action
On some instruments, such as certain guitars, the action can be adjusted by tightening screws at the bridge, which changes the height of the strings. Tune-o-matic bridges use small thumbwheels for this purpose; sometimes these are accompanied or replaced by flat-head screw fitting. On other instruments, changing the action is more difficult, involving the removal of entire pieces from the instrument.
The action on a guitar is also slightly affected by the adjustment of the truss rod. Tightening the truss rod gives the neck a backward bow and tends to lower the action, and loosening the rod gives the neck a forward bow, giving a higher action.
Action on a guitar is usually measured at the 12th fret. It is recommended that the action on an electric guitar should be 1/16" (1.6mm) on the high E string and 3/32" (2.4mm) on the low E string when in standard tuning using standard gauge strings.
Ideally a straight neck works the best for guitar action, but in some cases it is better to have a slight forward bow or relief in the guitar neck. The neck straightness can be adjusted by tightening and loosening the truss rod.
Adjustment of the action should be done using all the aforementioned truss adjustments, in addition to modifying or adjusting any elements on the bridge of the guitar.