Niccolò Paganini wrote 24 Caprices for solo violin in 1802-1817, and his 21st caprice is widely regarded as the most challenging. It is the only one of the caprices in the minor key, and this gives it a distinctively dark and dramatic sound. It also features rapid fingering, complex harmonics, and a high level of technical difficulty. Paganini himself was known for his prodigious skills on the violin, and he was the only one who could perform all of his caprices. Even today, it is a rare violinist who can play the 21st caprice without any mistakes.