sostenuto pedal
The middle pedal on a grand piano that selectively sustains only the notes being held at the moment it is depressed, while leaving subsequently played notes unaffected.
In Depth
The sostenuto pedal (from Italian "sostenuto," sustained) is the middle pedal on most modern grand pianos. Unlike the damper pedal (right), which lifts all dampers and sustains all notes, the sostenuto selectively holds up only the dampers of keys that are already depressed when the pedal is engaged. This allows the pianist to sustain specific notes or chords while playing other notes normally — a capability that opens up textural possibilities impossible with the damper pedal alone. The mechanism was invented by Steinway & Sons in 1874 and remains standard on quality grand pianos, though many upright pianos substitute a practice mute pedal in the middle position. Relatively few composers before the 20th century wrote specifically for the sostenuto pedal, but modern composers have exploited it extensively. It is essential for performing works by Messiaen, Boulez, and other 20th-century composers who require specific notes to be held while others move freely. Many pianists underuse this pedal, and its capabilities are frequently misunderstood even by experienced players.
Many pianists go their entire careers barely touching the sostenuto pedal — it is probably the most underused feature on the modern grand piano, despite offering unique capabilities that neither of the other two pedals can replicate.