The absolutely inaudible is rejected from music during the period of Viennese Classicism in which every musical line is potentially or imaginatively … - Charles Rosen

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The absolutely inaudible is rejected from music during the period of Viennese Classicism in which every musical line is potentially or imaginatively audible, but it makes a dramatic reappearance in the music of Schumann. The most striking of many examples is one of the episodes in the Humoresk, the last of the great piano works of Schumann's early years. ... There are three staves: the uppermost for the right hand; the lowest for the left; the middle, which contains the melody, is not to be played. Note that the melody is no more to be imagined as a specific sound than it is to be played: nothing tells us that the melody is to be heard as vocal or instrumental. This melody, however, is embodied in the upper and lower parts as a kind of after-resonance-out of phase, delicate, and shadowy.