Gui·do d'A·rez·zo 
(gwēdō dä-rĕtsō) or Guido A·re·ti·no (är′ĕ-tēnō) 990?-1050.
(gwēdō dä-rĕtsō) or Guido A·re·ti·no (är′ĕ-tēnō) 990?-1050. Benedictine monk and music theorist who devised the four-line staff, thereby allowing precise musical notation.