This is a manuscript about the theory of music. It was copied probably in England at Christ Church, Canterbury, in the second quarter of the twelfth century. Its main focus is the mathematical basis of music, and the beautifully-drawn diagrams with their graceful arches illustrate the mathematical ratios which produce the various intervals in the musical scale. Sometimes these diagrams take on animal forms such as here. Boethius, the author of this work, was born about 480 into an aristocratic Roman family. He wrote a number of works but his treatise on music, De musica, was one of his earliest, written probably in the first decade of the sixth century.