Guio of Arezzo or Guido Aretinus or Guido da Arezzo or Guido Monaco or Guido D'Arezzo (991/992–after 1033) 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).