What are the mechanics behind the middle piano pedal? I have been using the middle pedal for a variety of music, it is wonderful, but I can't figure out how it works (I mean the mechanical process). A drawing or animation would be ideal.

I know that the middle pedal is very underused, and I've never seen a music sheet specifying that one should press the middle and not the damper pedal. Is there standard notation for it?

EDIT: I'm talking about the original middle pedal, invented by Steinway. I think it's also called sostenuto pedal.

  • The latter part would be considered a poll, i.e., Not Constructive, so I took it out. Interesting question though.
    – user28
    Jan 29, 2012 at 17:56
  • Since you're essentially asking two different questions here, I've edited your tags to match the mechanical side of things.
    – NReilingh
    Jan 29, 2012 at 22:15

3 Answers 3


There are two different ways that the middle pedal on American pianos works. This pedal is called the "Sostenuto" pedal and, unlike the Sustain pedal, does not sustain every note on the piano. This website gives great videos and explanations of each piano pedal.

  • On higher end pianos, the middle pedal (Sostenuto pedal), sustains only those notes which are depressed on the keyboard at the time when the pedal is pushed down. Once the pedal is depressed, the notes played thereafter are not effected by the pedal.
  • Lower end pianos have a similar acting Sostenuto pedal, which, however, is not a true Sostenuto pedal. On these pianos, the middle pedal, while depressed, undamps only the lowest strings on the piano.

When a composer asks the player to use the Sostenuto pedal, he or she will use any of the following:

  • S.P
  • Sost. Ped.
  • ThP. (from German "Tonhalte-Pedal")

Some pianos, instead of using a Sostenuto pedal, provide a "Practice pedal" as the middle pedal. This pedal dampens the strings beyond the left most "Soft pedal". Often, this pedal has a locking mechanism as well.

The best way to know which type middle pedal a piano has is to test it out and see which of the above options it is.

EDIT: This website gives pictures and discussion of how some Sostenuto pedals work.

EDIT #2: This video demonstrates the sostenuto pedal mechanism.

  • Thanks for the notation on the sost. pedal, Reina, but I know I have a sostenuto pedal, and I know how and when to use it. What I do not know is how it actually works, mechanically. Probably this is not very explicit in my question, I will edit it. Good answer though, +1! :)
    – Victor
    Jan 29, 2012 at 22:06
  • @Victor, I'm assuming the mechanics will differ slightly depending on the maker. Lots of times, things like this are under patent and so can't be copied exactly from piano maker to piano maker. Jan 30, 2012 at 3:35
  • Remarkable piece of engineering! Thank you very much for the schematics. Although it may be slightly different depending on the maker, I now understand how (and why) it works!
    – Victor
    Jan 30, 2012 at 9:48
  • And a third type of piano have the middle peddle as a switch to put something between the hammers and strings (I think) to turn it into an electric piano - very useful for when neighbours get tired of loud practicing.
    – rlms
    May 6, 2014 at 20:22
  • @Victor can you open your piano and try to see what happens when you use the pedal (or someone else does)?
    – Mr. Boy
    Jan 15, 2015 at 9:37

On professional quality pianos and mediocre quality grand pianos, the middle pedal maintains only the notes that were maintained when the middle pedal was being depressed. This helps in any passage where a note or some notes should be maintained while some notes far away should not be maintained throughout. On some mediocre or poor quality upright pianos and poor quality grand pianos, the middle pedal maintains only the notes in the left-most register. This helps in a passage where all the notes that should be maintained are in the left-most register while all the notes that should not be maintained throughout are in some other register. On some other upright pianos, the middle pedal inserts a piece of felt between the hammers and the strings in each register. This helps if others tend to feel uncomfortable hearing the practice, and if any of the strings that are going to be played are painfully out of tune. On yet other upright pianos, the middle pedal is a duplicate of the left pedal. Some middle pedals on upright pianos have a locking option.


There often is no notation — you just use the sostenuto pedal out of necessity. For example, in Rachmaninoff's Prelude in c♯ minor, you would use it on the low C♯, especially in measure 6. You could use the damper pedal, but that would make a sloppy mess.

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