On a grand piano:
right pedal - lifts the dampers, so the strings (all of them) are free to ressonate.
left pedal - moves the hammers to the right so that the hammer now hits only one (in some pianos 2) of the three strings for each key (bass keys have only one string, and extreme treble only two , though). Hence "una corda". It has the effect of thining the sound, not necessarily lowering the volume.
middle pedal - sostenuto pedal. Press a key and before releasing it, press the middle pedal. It will raise the damper corresponding to that single key, which is allowed to vibrate freely - but not the other strings.
Upright piano:
Right pedal - just as above.
Left pedal - brings the hammers closer to the strings, so that the cannot strike the strings at full speed. Therefore it "muffles" the sound. This is not an una-corda pedal – as in a grand piano – and does not even tries to imitate the effect.I have never heard before of an upright piano with an una-corda pedal – that is a mechanism that shifts the hammers sideways so that only a single string is hit – which of course does not mean that they do not exist.
Middle pedal (when there is one) - Brings a piece of felt between the strings and the hammers, and is intended for practice without disturbing the neighbours. Seriously diminishes the volume. On the Yamaha Silent series, the middle pedal once engaged, connects the keyboard to a digital piano, so that it is completely silent, and you play with headphones. Disengage and the keyboard is again concected to the hammers and strings and you have a real piano.
This is not a sostenuto pedal – as in a grand piano – and does not even tries to imitate the effect. Its fuction is of a completely different sort (it puts the piano in “practice without disturbing the neighbours” mode). I had never heard before of an upright piano with a sostenuto pedal – that is a mechanism that allows a single string to vibrate unimpeded, until I read nav and puma’s posts above about the Baldwin.
Best wishes,
Bernhard.