The sostenuto pedal or "middle pedal" keeps raised any damper that was raised at the moment the pedal is depressed. This makes it possible to sustain some notes (by depressing the sostenuto pedal before notes to be sustained are released) while the player's hands are free to play other notes. This can be useful for musical passages with pedal points and other otherwise tricky or impossible situations. The sostenuto pedal was the last of the three pedals to be added to the standard piano, and to this day, many pianos are not equipped with a sostenuto pedal. (Almost all modern grand pianos have a sostenuto pedal, while most upright pianos do not.) A number of twentieth-century works specifically call for the use of this pedal. This pedal is often unused in modern music.
The bass sustain pedal can be found on many uprights and baby grands in place of the sostenuto pedal, which lifts all the dampers in the bass. It works like the damper pedal, but only affects the lowest notes.
Some upright pianos have a practice pedal or celeste pedal in place of the sostenuto. This pedal, which can usually be locked in place by depressing it and pushing it to one side, drops a "muffler" (or strip of felt) between the hammers and the strings so that all the notes are greatly muted; a handy feature for those who wish to practice in domestic surroundings without disturbing the neighbors. The practice pedal is rarely used in performance.
So many times piano pedals look the same, but have different functions. A few compositions that demonstrate the sostenuto pedal are Beethoven's "Moonlight Sonata (FULL) - Piano Sonata No. 14", Chopin's "Prelude in D Flat Major RAINDROP" and "Mine" by George Gershwin and Ira Gershwin 1933.