Sostenuto

In music, sostenuto (/ˌsɒstəˈnuːtoʊ/, Italian: [sosteˈnuːto]; Italian 'sustained') is usually a very legato style in which the notes are performed in a sustained manner beyond their normal values. It may imply a slowing of tempo.

Sostenuto pedal

The sostenuto pedal is the pedal in the middle under the keyboard of the grand piano

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"). The right pedal is the sustain pedal, which undamps all the strings on the piano, while the left pedal is the soft pedal.

On some upright pianos and lower-quality grand pianos, the middle pedal is not a sostenuto pedal but instead sustains all notes in the bass register. On other uprights, the middle pedal is a practice pedal (with a locking option), which makes the sound even quieter than 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. Not all pianos have a sostenuto pedal, however. Some older, or modern low-quality uprights do not have this option.