Denis Hirschfeldt provides more detailed information than I had previously had access to about the origin of the sol-fa syllables. He notes that the syllables come from a hymn for St. John the Baptist, which Britannica tells me was composed by Guido d'Arezzo (apparently to help teach people to sight-read music). (I keep trying to translate "Guido d'Arezzo" as "Guido with rice"...) The hymn starts like this:
UT queant laxis
LAbii reatum, Sancte Ioannes.
Each successive line starts one note higher on the scale (with a half step between mi and fa, and whole steps for the other intervals).
The details of the hexachord system remain beyond my comprehension, but the gist of it seems to be that you could apply the series of six notes at different (overlapping) places on a scale in order to more easily hear and sing the correct note relationships. For instance, you could start a song with ut corresponding to a certain pitch; if you wanted to go higher than la from that pitch, you could switch to a new hexachord, with the new ut being the same note as the old sol, or sometimes the old fa (depending on where you wanted the half step to end up). Eventually someone decided that it made more sense to have a set of seven relative pitch names so that each new ut was a full octave higher (or lower) than the original one. Denis Hirschfeldt adds, quoting from the Catholic Encyclopedia, that in 1599 one Erich Van der Putten suggested using "bi" from "labii" as the seventh note, but most others preferred "si," from the initials of Sancte Ioannes.
(Thanks also to Jere7my Tho?rpe, who provided some of this information in slightly different form around the same time.)
The coolest part of all this is the word "gamut." According to Britannica, the lowest note in use in Guido's time was the G an octave and a half below middle C; this note was called "gamma." Due to the overlapping hexachord system, ut could fall on any of several notes in the scale, so "gamma-ut" specifically meant the ut corresponding to that low G. The word "gamma-ut" later came to mean the entire musical scale, from gamma-ut up to the highest note used; eventually the word was shortened to "gamut."
Britannica also notes that there have been other solmization systems in other parts of the world, including India ("using the syllables sa, ri, ga, ma, pa, dha, ni"), China, and ancient Greece.
(Last updated: 20 April 1999)