In the lowest hexachord, the B is natural, in the second hexachord there is no B and in the third hexachord, the B is flattened. Our sign (♭) for flat comes from the fact that this B was called a round B and the sign (♮) for natural was called a square B. The sharp (♯) came from the natural and both meant at first raising the tone a half-step.
Guido once heard the monks in the monastery of Arezzo singing a hymn in honor of St. John the Baptist. He noticed that each line of the Latin poem began on ascending notes of the scale,—the first line on C, the second on D, and so on up to the sixth on A. It gave him the idea to call each degree of the hexachord by the first syllable of the line of the Latin hymn, thus:
Utqueant laxis,
Resonare fibris,
Mira gestorum,
Famuli tuorum,
Solve polluti,
Labia reatum.
Hymn to St. John the Baptist
