Most pianos have just two pedals — one for loud, one for soft. But sometimes you see a piano with three pedals. What does the third one do — make the music more … middle of the road, somehow? And why don't all pianos have one?
Silly boy. Let’s start at the beginning. The right-hand pedal is the “forte” pedal, also called the sustain or loud pedal. In ordinary piano playing, dampers plop down on the piano strings after you let up on the keys, stifling the earlier notes and making the new notes sound more distinct. On occasion, however, you want a richer, more resonant sound, so you depress the forte pedal. This prevents the dampers from falling, and all notes continue to resound until the pedal is released.
The left-hand pedal is called the “una corda” or soft pedal. Most notes in a piano are produced by two or three parallel strings tuned in unison. The una corda pedal shifts the piano’s innards over so that the hammers only strike two strings rather than three (or one rather than two), reducing the volume. In many upright pianos, the una corda is replaced by the “piano” pedal, which shortens the distance traveled by the hammers, another way of reducing volume.
The mysterious third or middle pedal is usually the “sostenuto.” When you press down a key or keys, then depress the sostenuto pedal, it will sustain those notes — and only those notes — until you let the pedal up. This allows you to play a note in the bass, for instance, and then move both hands up to the treble for a few bars while the bass note continues to sound. In effect you have an extra hand to work with. But it’s the sort of thing you only need with keyboard-spanning music like that of Debussy or Ravel. For the average Saturday night tinkler, two pedals will do just fine.
Piano making being an idiosyncratic business, there are many variations on the preceding. Sometimes there’s a modified sostenuto that only works for the bass notes. Sometimes it’s replaced by an alternative soft pedal called the “celeste.” And sometimes … but let’s quit while we’re ahead.
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