Whether his sonatas of the Vienna period are solo or accompanied by other instruments, they have only three movements. He first sought beauty of melody, for song was to him the foundation, the highest expression of music. Therefore the themes were carefully sung, and the second subject was made of more importance by him than by his predecessors. Often the chief effect in his sonata movements as in his concertos is gained by the delivery of a sustained melody, and these melodies written for his own hands show the influence of the peculiar characteristics of his own performance. Frequently in the elaboration of the themes he introduced new melodies, so that we find Dittersdorf complaining of the prodigality of the composer, who "gives his hearers no time to breathe." When he used polyphony, it was not to display pedantry but to accentuate the beauty of the themes.
The slow middle movements are in song form, and are full of emotion and tender grace; eminently spontaneous, and coming from the heart. The final movements are generally the weakest. They show the facility with which he wrote, and their gayness often approaches triviality. Passing over the pianoforte compositions for two performers and for two pianofortes--not that they are unworthy of attention-- we come to the sonatas with violin accompaniment, which, during the Vienna period, were, many of them, written for pupils. They are characterized by beautiful melodies and bold harmonies rather than by any great depth or exhibition of scholarship. The violin part is independent, and not an accompaniment, as was usual at the time. The trios or terzets for pianoforte, violin and 'cello were chiefly written for amateurs to play in musical parties. Violoncellists of any force were rare in these circles, and it is not unlikely that this was a serious hindrance to Mozart's further development of the trio. Far greater in breadth of design and in thematic elaboration are the two pianoforte quartets (1785 and 1786). The trios were written for social purposes, and brilliancy was perhaps too much cultivated; but in these quartets passion enters, strong and fierce and bitter. In 1784 Mozart wrote his father that his quintet in E-flat major for pianoforte, oboe, clarinet, horn, and bassoon, which was received with great applause in a concert given by him in the theatre, was the best thing he had ever written, and he chose it to play before Paesiello. It is certainly a composition of remarkable beauty, not so much on account of its thematic invention as for its intimate knowledge of the peculiarities of the different instruments and for the balance of euphony preserved throughout. The pianoforte concertos, of which seventeen were written in Vienna, were, as a rule, intended for his own concert use. He described the first three as "a happy medium between too easy and too difficult." He added in this letter to his father, that "even ignoramuses will be pleased with them without knowing why." Two years later (1784) he wrote, "I cannot make a choice between the two concertos in B-flat and D. Either one will make the player sweat." The distinguishing merit of these compositions for pianoforte and orchestra, unjustly neglected in these days, is the combination of the two different forces, while these forces at the same time preserve their individuality. Instead of a duel to the death between the instrument and the orchestra, there is a generous appreciation of the qualities and limitations of the pianoforte, which in Mozart's time was still weak in mechanism. Therefore one gives way to the other for the effect of the whole. The orchestra enters not to crush but to support. Often the pianoforte part seems absurdly simple, but a closer investigation will show that this simplicity is most artfully designed and intended. Seldom are important themes given to the pianoforte or orchestra alone; they are shared generously. And no words can reproduce the colors of the orchestral tone-paintings, or describe the marvelous results gained by simple means and an unerring instinct. The first movements are in the sonata form, but there is a certain freedom, and the proportions are on a larger scale. There is a cadenza, invariable, at the conclusion, and Mozart in his concerts excited wonder by his improvisations. The cadenzas published were for the use of pupils. The second movement is in song-form, full of sentiment, often romantic, the expression of temperament; the song is sometimes varied. The last movement is generally in rondo form, and the influence of the dance is strongly marked. These movements are gay and graceful, and occasionally there is a touch of Haydn's humor. The greatest of these concertos are perhaps those in D minor (K. 466), C (467), C minor (491) and in C (503).