As a theorist, he was an expert in Schenkerian analysis, and he was the co-author (with Carl Schachter) of one of the standard theory textbooks used throughout the United States, Harmony and Voice Leading (first published in 1979).
As an expert in Schenkerian analysis, he contributed to the development of a widely accepted theory of harmony and voice leading, as evident from his co-authorship with Carl Schachter on the influential textbook Harmony and Voice Leading (first published in 1979).