In music, sostenuto is a term from Italian which means "sustained," and occasionally also implies a slowing of tempo. It usually refers to a style of playing rather than a tempo. For instance, if the tempo marking of a piece is Adagio Sostenuto, the tempo would be slow (Adagio), and the sostenuto part of the marking tells the player to hold notes longer than they normally would, and play phrases in a very legato style.
On a modern grand piano with three pedals, the middle pedal is usually a sostenuto pedal. It sustains only those notes which are depressed at the same time that 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, 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.
Note that the sostenuto pedal should not be confused with the much more commonly used sustain pedal, which undamps all the strings on the piano.
