In the course of the nineteenth century, four-hand piano playing emerged across Europe as a popular pastime of the well-heeled classes and of those looking to join them. Nary a canonic work of classical music that was not set for piano duo, nary a house that could afford not to invest in them. Duets echoed from the student bedsit to Buckingham Palace, resounded in schools and in hundreds of thousands of bourgeois parlors. Like no other musical phenomenon,...