Guido of Arezzo or Guido Monaco (995-1050) is regarded as the inventor of modern musical notation (staff notation) that replaced neumatic notation.
Guido was a friar of the Benedictine order from the Italian city-state of Arezzo. He noted the difficulty that singers had in remembering Gregorian chants.
He developed new technologies for teaching, including the staff notation and the "do-re-mi" scale, in which the name of the single notes were taken from the initial syllables of the seven verses of a hymn, Ut queant laxis (at the beginning, "do" was called "ut").
The simple placement of lines allowed those reading musical notation to know where on the scale a particular note should be sung, moving from a relative scale (useful to those needing a reminder of where to sing) to an absolute scale.