Soft pedal
The soft pedal or una corda pedal (Italian for 'one string'), is one pedal on a piano, generally placed leftmost among the pedals. On a grand piano this pedal shifts the whole action (including the keyboard) slightly to the right, so that the hammers, which normally strike all three of the strings for a note, strike only two of them. This softens the note and also modifies its tone quality. Tone quality is also affected by forcing the remaining two strings being struck to make contact with a part of the hammer felt which is not often hit (due to the whole action being shifted); this results in a duller sound, as opposed to the bright sound which is usually produced (due to the felt being hardened from regular use).
Upright and digital pianos[edit]
On most upright pianos, the soft pedal operates a mechanism that moves the hammers' resting position closer to the strings. Since the hammers have less distance to travel this reduces the speed at which they hit the strings, and hence the volume is reduced, but this does not change tone quality in the way the una corda pedal does on a grand piano. Only a minority of upright pianos have una corda action.
Digital pianos often additionally use this pedal to modify non-piano sounds such as the organ, guitar, or saxophone in ways appropriate to those instruments' playing techniques. Pitch bends, Leslie speaker speed, vibrato, and so forth can thus be controlled in real-time, analogous to the "modulation wheel" on a synthesizer. The pedal is still sometimes called the soft pedal because of its position, but it may have another name like modulation pedal.
Musical notation[edit]
The use of the soft pedal is generally notated with the words una corda (Italian for 'one string') or due corde (Italian for 'two strings') to show when the pedal should begin being used, and tre corde or tutte le corde ('three strings' or 'all the strings') for when it should be released. There is discretion for the performer in its use, however, and it can be used when there is no notation when the performer believes its timbre or quietness is called for by the piece.