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The action of an instrument plucked by hand is the distance between the fingerboard and the string. In keyboard instruments, the action is the mechanism that translates the motion of the keys into the creation of sound (by plucking or striking the strings).
In a harpsichord, the main part of the action is a jack, a vertical strip of wood seated on the far end of the key. At the top of the jack is mounted a hinged tongue bearing a plectrum. When the key is pressed and the jack rises, the plectrum plucks the string. When the key is released and the jack falls back down, the tongue permits the plectrum to retract slightly, so that it can return to its rest position without getting stuck or plucking the string again on the way down. The jack also bears a damper, whose purpose is to stop the vibration of the string when the key is released. For full description and diagrams, see Harpsichord.
In a piano, the action is a mechanical device, made mostly of hardwoods, that serves several purposes. By means of various levers, it permits a small motion of the key to be translated into a large motion of the hammers that strike the strings. The action also permits a hammer to recoil from the string instantly so as not to damp its vibration, and it also prevents the hammer from bouncing up and down, striking the string multiple times. Piano actions, even in their original version invented by Bartolomeo Cristofori, tend to be quite complex and have been subject of ingenious inventions and refinements throughout their history. Different actions are used in grand and vertical pianos. For full discussion, including diagrams, see Action (piano) and piano.
Instruments plucked by hand
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. On a steel string acoustic guitar, for example, the action can be adjusted by carefully sanding down the guitar's saddle so that the strings sit closer to the fretboard.
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.