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In music, sostenuto is a term from Italian that means "sustained". It occasionally implies a slowing of tempo, though more often it refers to a very legato style in which the notes are performed in a sustained manner beyond their normal values.
On a modern grand piano with three pedals, the middle pedal is usually a sostenuto pedal. It sustains only those notes that are being held down when the pedal is depressed, allowing future notes played to be unaffected. It is commonly abbreviated "S.P.", "Sost. Ped.", or "ThP." (from the German equivalent "Tonhalte-Pedal").
On some upright pianos and lower-quality grand pianos, the middle pedal sustains all notes in the bass register, but this is not a true sostenuto pedal. On other uprights, the middle pedal is a practice pedal (with a locking option), which makes the sound extremely quiet beyond the standard soft pedal. This is often achieved by dropping a felt cloth between the hammers and the strings when the practice pedal is depressed.
The sostenuto pedal should not be confused with the much more commonly used sustain pedal, which undamps all the strings on the piano.