U is for Una corda: the pedals
The left hand (‘soft’) pedal is known as the una corda (Italian for ‘one string’). On modern grand pianos, it shifts the keyboard to the side so that fewer strings are struck; on uprights, it moves the hammers closer to the strings.
The right hand one is the sustaining pedal (sometimes improperly called the ‘loud pedal’) which raises the dampers from the strings, allowing them to vibrate freely until the pedal is released.
The (middle) sostenuto pedal on a grand allows the sustaining of certain notes or chords while others continue to be damped. On uprights, the middle pedal is usually a ‘practice’ pedal, muffling the strings to give some respite to one’s neighbours.