sostenuto pedal

The middle pedal on a grand piano — sustains only the notes that are depressed at the moment the pedal is engaged, leaving subsequent notes unaffected.

Allows a bass note or chord to sustain while the upper hands play freely without the pedal. Used in specific works (e.g., Debussy, Ravel, Bartók) that require this selective sustain. Not present on most upright pianos.