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On every acoustic instrument is played, it does not just produce the frequency is played (the fundamental) but also the sequence of frequencies as the series of integers multiple of a fundamental. It also applies to the piano and has some special effects to play around.
Diamond notehead indicates pressing the key silently. If we silently press a key (release the damper on the string) an octave of the note you strike, you can hear the octave-higher note is ringing/ haunting. This is the result of harmonic series, the natural vibration of the string. This also works when you silently press the key an octave and a fifth higher. I can play this effect around, the notes or chord I want to play press silently. Then, I strike the keys below, the chord is isolated by the harmonic series.
Limitation
This effect is effectively below the C5 of the dampen key since the stings on the high range are too short to pick up the vibration below and sustain very little. The harmonics that can be used are mostly the first (an octave above) and second partials (an octave and fifth above). Since the piano is tuned in equal temperament. Except for the first there partial, the rest are quite out of tune compare with the ET. At the same time, the higher partials it goes, the softer in volume.
