A Dictionary of Music and Musicians/Fundamental Bass
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FUNDAMENTAL BASS is the root note of a chord, or the root notes of a succession of chords, which might happen to be the actual bass of a short succession of chords all in their first positions, but is more likely to be partly imaginary, as in the following short succession of complete chords, which has its fundamental bass below on a separate stave:—
Rameau was the first to develop the theory of a fundamental bass, and held that it might 'as a general rule proceed only in perfect Fourths or Fifths upwards or downwards.' Helmholtz defines it as 'the compound tone which represents the chord, as distinguished from its bass, that is, the tone which belongs to the lowest part.'[ C. H. H. P. ]