The sonata in B-flat, Kv.454 dates from April 1784. It was written for the Italian violinist Regina Strinacchini and was premiered by her and Mozart for the Emperor Joseph II. Curiously only the violin part was written out and Mozart himself played the whole part from memory. Later that year the sonata was printed by 'Torricella and in 1787 was reprinted by Artaria, who had printed sonatas (Kv.296, 378 etc) before.
The whole sonata can be seen as a chamber music piece, it is more complicated in style, the instruments are perfectly balanced in importance and the sonata, curiously, changes instruments during one melody. The complexity from the chamber-music performance lays exactly in these places where the integration of ideas and colors have to match perfectly.
The first movement starts with a largo introduction. Curiously, Mozart wrote very accurately that the piano should keep slightly longer than the violin. We should definitely take into consideration what Mozart intended with this. One can only hint at 'why' he would have written it but when we take into consideration that ink was rather expensive during his age, we have to understand that he had a clear idea about keeping the piano longer than the violin and that this was important for him.
In the Allegro, that follows the Largo-introduction, Mozart starts with both instruments with the melody, though the piano plays an octave lower. It has a very happy character with lots of surprises like for example the D-flat in bar 25. The first theme has a very rhythmic character and gets contrasted by the more melodic part starting from bar 50. The development is rather short with a lot of chromatism. The coda shows simply how much fun this movement is by first making the instruments play together and instantly changing that the violin plays between the notes as a variation, making it look like a mistake from the performers.
The second movement is an Andante that constantly changes between the main line and the 'accompanying' line between the instruments. There are many moments in which both instruments play exactly the same rhythmic figures which creates a very special atmosphere. In the middle section, the minor part and the many modulations bring us back to the main theme.
The rondo form of the third movement is written in an Alla-breva Allegretto. It is a very happy movement with lots of virtuosity from both instruments. Again both instruments are changing rapidly between main and accompany lines and each instrument has the chance to make a small cadensa. The coda, with all its virtuosity in both instruments bring the piece to a typical 'Mozartian' conclusion.