Mozart, along with Haydn and Beethoven, was a central representative of the classical style. His works spanned the period during which that style transformed from a predominantly simple musical language, as exemplified by the stile galant of his contemporaries such as Sammartini and Johann Stamitz, to a mature style which began to incorporate some of the contrapuntal complexities of the late Baroque, complexities against which the galant style was a reaction. Mozart’s own stylistic development closely paralleled the maturing of the classical style as a whole. In addition, he was a prolific composer and wrote in almost every major genre, including symphony, opera, the solo concerto, chamber music including string quartet and string quintets, and the keyboard sonata. While none of these genres were new, the piano concerto was almost single-handedly developed and popularized by Mozart. Mozart also wrote a great deal of religious music including masses. He also composed many dances, divertimenti, serenades, and other forms of light entertainment.Â

The central traits of the classical style can all be identified in Mozart’s music. Clarity, balance, transparency, and uncomplicated harmonic language are his hallmark, although in his later works he explored chromatic harmony to a degree rare at the time. Mozart is commonly named along with Schubert as having a gift for pure, simple, and memorable melody, and to many listeners this is his most definitive characteristic.

From his earliest life Mozart had a gift for imitating the music he heard; since he travelled widely, he acquired a rare collection of experiences from which to create his unique compositional language. When he went to London as a child, he met J.C. Bach and heard his music; when he went to Paris, Mannheim, and Vienna, he heard the work of composers active there, as well as the spectacular Mannheim orchestra; when he went to Italy, he encountered the Italian overture and the opera buffa, both of which were to be hugely influential on his development. Both in London and Italy, the galant style was all the rage: simple, light music, with a mania for cadencing, an emphasis on tonic, dominant, and subdominant to the exclusion of other chords, symmetrical phrases, and clearly articulated structures. This style, out of which the classical style evolved, was a reaction against the complexity of late Baroque music. Some of Mozart’s early symphonies are essentially Italian overtures, with three movements running into each other; many are “homotonal” (each movement in the same key, with the slow movement in the tonic minor). Others mimic the works of J.C. Bach, and others show the simple, rounded binary forms commonly being written by composers in Vienna.

As Mozart matured, he began to incorporate some features of the abandoned Baroque styles into his music. For example, the Symphony No. 29 in A Major K. 201, uses a contrapuntal main theme; in addition, in it he began to experiment with irregular phrase lengths, something a galant composer such as Sammartini never did. Some of his quartets from 1773 have fugal finales, probably influenced by Haydn, who had just published his opus 20 set. The influence of the Sturm und Drang (”Storm and Stress”) period in German literature, with its brief foreshadowing of the Romantic era to come, is evident in some of the music of both composers at that time.

In Mozart’s hands, sonata form transformed from the binary models of the Baroque into the fully mature form of his later works, with a multiple-theme exposition, extended, chromatic and contrapuntal development, recapitulation of all themes in the tonic key, and coda.

Throughout his life Mozart switched his focus from writing instrumental music to writing operas, and back again. He wrote operas in each style current in Europe: opera buffa, such as The Marriage of Figaro or Così fan tutte; opera seria, such as Idomeneo or Don Giovanni; and Singspiel, of which Die Zauberflöte is probably the most famous example by any composer. In his later operas, he developed the use of subtle and slight changes of instrumentation, orchestration, and tone colour to express or highlight psychological or emotional states and dramatic shifts. Here his advances in opera and instrumental composing interacted upon one another. The increasing sophistication of his use of the orchestra in his symphonies and concerti served as a resource in his operatic orchestration, and his developing subtlety in using the orchestra to psychological effect in his operas was reflected in his later non-operatic compositions.

Many important composers since Mozart’s time have worshipped or at least been in awe of Mozart. Rossini averred, “He is the only musician who had as much knowledge as genius, and as much genius as knowledge.” Beethoven’s admiration for Mozart is clear: Beethoven used Mozart as a model a number of times: Beethoven’s A-major Quartet from Op. 18 makes careful use of Mozart’s Quartet in A K. 464. Beethoven even copied out most of the Mozart quartet before he wrote his own A-major quartet, just to figure out how Mozart put the music together. A plausible story–not corroborated–has one of Beethoven’s students looking through a pile of music in Beethoven’s apartment. The student pulls out the Mozart A-major Quartet, Beethoven notices, and says, “Ah, that piece. That’s Mozart saying ‘here’s what I could do, if only you had ears to hear!’”; Beethoven’s own Piano Concerto No. 3 in C minor is an obvious tribute to Mozart’s Piano Concerto No. 24 in C minor, and yet another plausible–if unconfirmed–story has Beethoven at a concert with his sometime-student Ries. They’re listening to Mozart’s C-minor concerto Piano Concerto No. 24. The coda of the last movement is quite unusual, for various reasons, and when it arrives, Beethoven supposedly says to Ries “We’ll never think of anything like that!” Beethoven’s Quintet for Piano and Winds is another obvious tribute to Mozart, similar to Mozart’s own Quintet of the same kind. Beethoven also paid homage to Mozart by writing sets of variations on several of his themes: for example, the two sets of variations for cello and piano on themes from Mozart’s Magic Flute, and cadenzas to several of Mozart’s piano concertos, most notably the Piano Concerto No. 20, K466 (see below for this system and an explanation). After the only meeting between the two composers, Mozart noted that Beethoven would “give the world something to talk about.” As well, Tchaikovsky wrote his Mozartiana in praise of him; and Mahler died with the name “Mozart” on his lips. The variations theme of the opening movement of the A major piano sonata (K331) was used by Max Reger for his Variations and Fugue on a Theme of Mozart, written in 1914 and among his best-known works in turn.

In the decades after Mozart’s death there were several attempts to catalogue his compositions, but it was not until 1862 that Ludwig von Köchel succeeded in this enterprise. Many of his famous works are referred to by their Köchel catalogue number; for example, the Piano Concerto in A major (Piano Concerto No. 23) is often referred to simply as “K.488″ or “KV.488″. The catalogue has undergone six revisions.