Guido of Arezzo (also Guido Aretinus, Guido da Arezzo, Guido Monaco, or Guido d'Arezzo, or Guy of Arezzo) was a music theorist of the Medieval era. He is regarded as the inventor of modern musical notation (staff notation) that replaced neumatic notation; his text, the Micrologus, was the second-most-widely distributed treatise on music in the Middle Ages (after the writings of Boethius).