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How does the middle pedal on a grand piano work?
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On a grand piano, the middle pedal is typically known as the sostenuto pedal. Its function is to selectively sustain specific notes while leaving others unaffected.1
When you press this pedal, it lifts the dampers only for the keys that are currently being held down at that exact moment. This allows those specific notes to continue ringing, while any subsequent notes played will not be sustained by the pedal, as their dampers remain engaged. This mechanism provides pianists with greater control over music by allowing them to isolate and sustain particular tones or harmonies without blurring the entire performance.12
While this feature is a standard component of most grand pianos, it is also found on some upright models.2