A lovely tribute — and a musical riddle. Elgar’s Enigma Variations was, in effect, his version of a mixtape: each movement was given as a tender portrait of a personal friend. These powerful and emotional tokens of friendship have become extraordinarily popular — the “Nimrod” variation is often used as a song of national mourning, and was played at Princess Diana’s funeral. Not content with writing a beautiful piece, Elgar claimed that there was a musical mystery hidden deep in this work. Many claimed to have solved it, but no one knows for certain.