Peter Westergaard challenged the traditional view of the primacy of this concept in a 1975 "introduction" to his alternative theory. Edward Aldwell and Carl Schachter wrote an influential textbook on this concept "and voice leading." The most popular textbook on this subject has been through four editions beginning in 1941 and was written by Walter Piston. Novel ideas about inversions and progressions were laid out in a seminal 1722 treatise on this subject written by Jean-Philippe Rameau. This concept is sometimes called "vertical music," in contrast to the "horizontal music" of the melody. For 10 points, give this term for the simultaneous 
Peter Westergaard challenged the traditional view of the primacy of this concept in a 1975 "introduction" to his alternative theory. Edward Aldwell and Carl Schachter wrote an influential textbook on this concept "and voice leading." The most popular textbook on this subject has been through four editions beginning in 1941 and was written by Walter Piston. Novel ideas about inversions and progressions were laid out in a seminal 1722 treatise on this subject written by Jean-Philippe Rameau. This concept is sometimes called "vertical music," in contrast to the "horizontal music" of the melody. For 10 points, give this term for the simultaneous sounding of multiple notes.