Why did Mozart and Haydn study Johann Joseph Fux’s Gradus ad Parnassum?

Wolfgang Amadeus Mozart and Joseph Haydn studied Johann Joseph Fux’s Gradus ad Parnassum (1725) because it served as the standard, authoritative textbook for learning counterpoint throughout the 18th century.1

Fux’s treatise was designed to systematize contrapuntal practices by abstracting them from 16th-century models. During the Classical period, while musical textures were increasingly dominated by homophony and accompanied melody, composers still relied on this work to master the "learned style" of polyphonic writing. By studying the text, composers like Mozart and Haydn gained the technical proficiency necessary to incorporate complex contrapuntal elements—such as fugues—into their own works, effectively bridging the gap between traditional polyphonic techniques and the evolving styles of their era.213