This is from "Basic Music Theory on the World Wide Web" by Neil Hawes.

"The tonic sol fa is one method of naming note pitches - see history of notation for more details. Most people know of it from the song "Doh re me", from the 1959 film "The Sound of Music" by Rogers and Hammerstein. However, the origins of the tonic sol fa are not in the 1950s, but the eleventh century!

A Benedictine monk, Guido of Arezzo, took the first notes of each line of a Latin hymn, written around 770 A.D. which happen to be the first six notes of a major scale, and used the syllables of the Latin words that were sung on those notes to represent the notes of the scale. This gave "ut", "re", "mi", "fa", "sol" and "la" to represent the first six notes of a major scale and these names are still used (I believe) in the French system of naming notes ("ut" for C, "re" for D etc.). In Italy, the "ut" was changed to "do", being the first syllable of "Dominus". "si" was added later as the seventh note of the scale, being the initial letters of the name at the end of the hymn (which in fact does not use the seventh note of the scale because it was probably not part of the normal scale at the time).

"si" was much later changed to "te" by a Miss S. A. Glover and John Curwen (1816-1880), a Congregational minister in England, so that each degree of the scale would have a unique single letter abreviation used for written notation. This was the start of the "movable doh" method of teaching which lasted in the UK for a hundred years.

The system can be used (and was in common use in the early part of this century) to teach the notes of the major scale, and the intervals between them. Sometimes, pieces were written with the tonic sol fa names written underneath.

An apparent disadvantage is that chromatic notes cannot be notated, and only one octave can be described. However, there have been attempts to overcome these problems. For different octaves, various schemes have been tried using ticks, or different cases or print styles to indicate different octaves. For chromatic notes, the following is sometimes seen when a line of music is modulating, and the possibilities are as follows:

For sharpened notes: de, re, fe, se and le (pronouced with a short vowel).

For flattened notes: ra, ma, la, ta (pronounced with a nasal "aw" sound, I'm told).

So, for example, the minor sixth from doh would be la (pronouced law), as opposed to lah for the major sixth. Full chromaticism is not needed, because a tune is normally re-notated into the new key, by re-positioning the doh, even if the new key only lasts for a few bars. I would suggest that using Tonic Sol-fa with full chromaticism would lose the advantages of simplicity and readability."

Cheers!
Ian