The set of violin sonatas of Beethoven is, debatably, one of the most performed and popular sets in the repertoire for violin and piano. The set contains 10 sonatas in which the first nine were written within 6 years (1797-1803) and the last sonata, op.96 in 1812.
When one looks closely at these works, one can see a huge development in composition style, inventiveness between movements and the use of instruments. The violin sonatas of the 18th century were mainly 'piano sonatas with the accompaniment of the violin' but Beethoven changed this with the climax in the 9th sonata, op.47, commonly known under the name 'Kreutzer' sonata.
The composers of the classic period mostly knew how to play multiple instruments. Mozart knew how to play the violin (and viola) next to his main instrument; the fortepiano. Haydn had training in, among others, violin and harpsichord and Beethoven was, though not considered to be a decent player, trained in the violin as well as the piano and played as an orchestra-member in two orchestras to financially support his brothers. This made it possible for those composers to write compositions with a full understanding of the instruments. One can see this not only in the quality but also in the quantity of the sonatas that were written, 36 piano-violin sonatas by Mozart and 10 by Beethoven.
Beethoven sticks in general with the classical tradition of keeping the pieces in a Major key. There are two exceptions; the Sonata no.4, op.23 in A minor and the Sonata in C-minor op.30 no.2. Also, the melodic figures are often very classically build in the sense that they are of the structure of 4+4 bars; making the 8 bar phrase.
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Even though these sonatas are written, except his tenth sonata, in the second and so-called 'Heroic' period, one can see very clearly the the development towards the style that later would become so apparent in his work. The off-beat Sforzandi, which give such a feeling of change in metre, the big crescendo's with sudden changes to very piano phrases, the big differences in characters between the phrases and sections, one can see it all in his violin-piano sonatas. It is Beethoven at his best, like for example in his string quartets.