Was the first composer to indicate dynamic markings in his music.
Is generally credited with developing a form of music commonly known today as "sonata-allegro form", most often used in the first movement of many symphonies and concertos. This form consists of four distinct sections: the exposition, in which the themes of the movement are stated; the development where these themes are developed further (broken down, fragmented, inverted, etc.); the recapitulation in which the themes are restated; and the coda, a closing statement. An introduction may or may not be included.
His more famous symphonies have appropriate nicknames. The "Farewell" symphony dropped a hint to Esterhazy that the musicians deserved a vacation; the "Clock" symphony suggests a ticking clock. The "Surprise" symphony (#94, in G Major) features a fortissimo chord in the second movement, the "surprise" written to make the ladies of the court jump in their seats.
Wrote some 40 operas, very few of which are performed today.
Though he and his wife, Maria Anna Keller, had no children, Haydn did have an illegitimate son, Alois Anton Nikolaus Polzelli, born April 22 1783, with soprano singer Luigia Polzelli.
His birthdate is often reported as March 31, 1732, but this is a fabrication of his well-meaning brother, Johann Michael Haydn, who did not want his famous sibling to be remembered as an "April's Fool".
For many years was mistakenly believed to be the composer of Leopold Mozart's "Toy Symphony".