E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Ut, re, mi, fa, sol, la, used by Guido dArezzo in the eleventh century for his system of hexachords. Hexachord means a scale of six notes. They are the first syllables of some words in the opening stanza of a hymn for St. Johns Day. Ut queant laxis re-sonare fibris, etc. Si, the seventh note, was not introduced till the seventeenth century. Originally thescale consisted of six notes only. (See Do.)
Auparavant on ne se servait que de six notes; et on remplacait le si au moyen de combinaisons appelees nuances.Bou llet: Dictionnarie des Sciences, p. 1523, col. 2.)