The middle piano pedal can sustain notes or quieten practice

An upright’s middle pedal may lower felt between hammers and strings for quiet practice. Yamaha describes that arrangement in its piano guide. On a grand, the same position commonly belongs to sostenuto, which has a different task: keeping selected notes ringing after the fingers leave the keys.

Kawai explains the practical distinction. Hold a low key, engage sostenuto, then release the key: its damper stays raised while later notes can still be played short. The right-hand damper pedal raises all the dampers instead. Choosing which sound continues is different from sustaining everything the pianist plays.

Even the upright example is not universal. Steinway describes a Mezzo Dolce middle pedal that brings the hammers closer to the strings while also lifting other action components and the keys. The company presents it as a way to play quietly; felt is not part of that explanation. A model’s specification is more useful than a function guessed from pedal position.

The linked live performance of Debussy’s La cathédrale engloutie is by Krystian Zimerman. It offers related piano listening, not a demonstration of a particular pedal. Listen to the duration of sounds without assuming that the recording identifies what his foot did. At an unfamiliar instrument, three pedals need not mean three familiar functions.