Two of the five concerts for violin by the most famous violin player that ever breathed. Paganini (1782 – 1840) is most famous for his Caprices, opus 1, but these concertos come a close second. In his works, he managed to push the limits of what is technically possible on a violin, and especially the first concerto is one that demands a very virtuous player.
An example of the demanded virtuosity might be that in the first concerto, the orchestral parts are written in E flat, with the soloist’s instrument tuned a semitone higher. That way the soloist is able to play more on open strings, where the orchestra has to rely on fingering to get the same. Open strings sound louder and more clean, and the result is that even though they play the same tune, the soloist naturally sounds louder.
Paganini did much of his composing on a guitar by the way. Most composers use a piano as a compositional aid.