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).
A renowned music theorist, his expertise lay in Schenkerian analysis, which he applied to a widely used textbook on harmony and voice leading that he co-authored with Carl Schachter, first published in 1979.