There are typically three pedals on a piano. From left to right, the first pedal is called the soft pedal (or una corda), the sostenuto pedal (usually found in American-made grand pianos), and the sustain pedal (or damper).
On some pianos the middle pedal is used as a muting function (not sostenuto), so the keys play a lot quieter when you hit them – and some pianos don’t have this middle pedal at all.
The una corda pedal was invented by Bartolomeo Cristofori (who also invented the piano in around 1700), which was the first stop mechanism used to modify the pianos sound. The common name used today for the una corda is the soft pedal, which isn’t a very accurate name because not only does it make the sound quieter, it also changes the timbre (character or quality) and tone, and not just the volume.
Cristofori’s original invention was operated with a hand stop, not a pedal. This stop was a knob on the side of the piano. When this was activated, the entire action of the hammers were moved to the right so that the hammers only hit one string (una corda) rather than two strings (due corda).
By the late eighteenth century pianos were built with three strings (for each note and hammer to hit), so that when this pedal was pressed you were often hitting two strings rather than just one. This made the sound very different to Cristofori’s original stop invention, and modern pianos still to this day only hit two strings when this pedal is pressed, which means we don’t have the same flexibility to change the tone like on the earlier pianos.
On a modern upright piano this is completely different, and isn’t truly a una corda pedal anymore as it doesn’t shift the hammers to the right. A more accurate name for this pedal is the half-blow pedal, which moves the hammers closer to the strings to leave less distance for the hammer to swing.
The very last pedal to be added to the modern grand piano was the middle pedal, or sostenuto pedal. This pedal, when pressed, allows the player to sustain selected notes, while the other notes remain unaffected. For example, you can play a note and then hold this pedal down, and only that note or set of notes you have played will be sustained. Anything else you then play isn’t sustained. This pedal is the least common, and isn’t used very often at all. There were only a few composers that used this pedal, like Claude Debussy, Maurice Ravel, and Charles Griffes.
It is very common nowadays to find the middle pedal missing on uprights and even grand pianos. And pianos that do have a middle pedal are not always the traditional sostenuto, as there are many other functions for this middle pedal. For example, the middle pedal can sometimes be another half-blow pedal, but it will slip into a groove to hold it in place so you don’t have to keep your foot down. The middle pedal may also sometimes lower a piece of felt in between the hammers and the strings to soften the sound, which is often used when practicing.
The most commonly used pedal is the sustain pedal, which is always on the far right. When pressing this pedal the dampers move away from the strings allowing the sound to continue vibrating after the hammer strikes. Without this pedal a pianist wouldn’t be able to play smoothly (legato), or it would be at the very least extremely difficult because you wouldn’t be able to hold all of the notes down for the sound to continue. The sustain pedal allows a pianist to play as many notes as they like, one after the other, and for these notes to continue until the pedal is released.