Before the Carlovingian era the practice of music was restricted to the singing schools founded for the preservation and propagation of Gregorian chant. But with the great impetus given to learning under Charlemagne the consideration of liturgic music passed to the monastery study. Music became a compulsory subject in the curriculum of the cathedral and monastery schools, and its theory as well as its practice received the attention of the learned monks and scholars. It was from this direction that the next recorded advances in musical art appeared. In the writings of these ecclesiastical musicians and scholars we find accounts of the clumsy, yet persistent efforts of the singers and theorists to break away from the prevailing monophony or unison chanting of Gregorian music and to improve upon current systems of notation. The Flemish monk Hucbald (who died about 930), in his _Musica enchiriadis_, described the earliest known efforts at polyphony, which he called Organum or Diaphony (See Vol. I, pp. 161 ff). Guido d’Arezzo (died about 1050), sometimes called ‘the father of music’ and undoubtedly the most impressive musical personality in the early part of the Middle Ages, probably originated the four-lined staff for indicating pitch relationships and invented solmization, a system of reading music through the association of tones with syllables that is the direct ancestor of our present-day systems of reading music by syllables (‘Tonic Sol-fa,’ ‘Movable Do,’ ‘Fixed Do’). He is credited by later writers with many innovations and discoveries which possibly belonged rightfully to talented and ingenious contemporaries who, however, did not succeed in stamping themselves on their own age as vividly as did this great singer and teacher. Franco of Cologne (died about 1200), in his famous treatise on Measured Music, gives a voluminous account of his own and contemporary thought about intervals, consonances and dissonances, time-values of notes, etc.