Ut queant laxis resonare fibrisThe ut being the conjunction "that." Gamut also was used for "range of notes of a voice or instrument" (1630s), also "the whole musical scale," hence the figurative sense of "entire scale or range" of anything, first recorded 1620s. When the modern octave scale was set early 16c., si was added, changed to ti in Britain and U.S. to keep the syllables as different from each other as possible. Ut later was replaced by more sonorous do (n.). See also solmization.
Mira gestorum famuli tuorum,
Solve pollutis labiis reatum,
Sancte Iohannes.