Guido Aretinus, a Benedictine monk who flourished at Arezzo, in Italy, during the 11th century, the first to promote the theoretical study of music; he is credited, amongst other things, with the invention of counterpoint, and was the first to designate notes by means of alphabetical letters, and to establish the construction of the stave.
Definition taken from The Nuttall Encyclopædia, edited by the Reverend James Wood (1907)
Guicowar * Guido Reni