Guido of Arrezo is a Benedectine monk from Italy and an Italian music theorist of the Medieval Era. While at Arrezo he develops new techniques and method in teaching music such as staff notation and the use of six note pattern aid "ut–re–mi–fa–so–la" which also means do–re–mi–fa–so–la).
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The gentlewoman will not be coaxed to reveal what she has heard, because she has "no witness to confirm my speech." There is only her word that Lady Macbeth has said what she has said, and the gentlewoman does not want to be responsible for whatever consequences may come of her reporting these things.