Joseph Haydn is one of the most recognisable composers of the Classical period, and is called the "Father of the Symphony" and "Father of the String Quartet". Haydn's musical practice formed the basis of development of tonality and musical form. Taking genres such as the symphony, which were at the time shorter and subsidiary to more important vocal music, and slowly expanded their length, weight and complexity. Haydn's work became central to what is described as sonata form, and his work was central to taking the binary schematic of what was then called a "melodie". It was a form divided into sections, joined by important moments in the harmony which signalled the change. One of Haydn's important innovations (adopted by Mozart and Beethoven) was to make the moment of transition the focus of great creativity. Instead of using stock devices to make the transition, Haydn would often find inventive ways to make the move between two expected keys.
He has influenced many composers, but the two that stand out are his good friend, Wolfgang Amadeus Mozart. In a lot of Mozart's compositions, you can hear definite characteristics from Haydn's work. He also influenced his student, Ludwig van Beethoven, a great deal.
The first great symphonist, he composed 106 symphonies, including the popular last 12 "London symphonies" (1791 – 95). He virtually invented the string quartet, and his 68 quartets remain the foundation of the quartet literature. His choral works include 14 masses and the oratorios The Creation (1798) and The Seasons(1801). He also wrote 47 piano sonatas and more than 125 beautiful works for the cello-like Baryton.