As originally conceived, solfege was a movable do system. Whatever pitch was the tonic would be assigned the syllable “do” and the other syllables, re, mi, fa, so, la, and ti followed upward by step. In today’s usage, these movable do syllables are referred to as tonal syllables. They are called tonal syllables because they identify pitches by harmonic function, not by name or location on the staff. The tonic pitch is always “do” regardless of the key, and so also with the other pitches. Another way of putting is that the function (tonic, dominant, sub dominant, etc.) is always called by the same syllable, but the pitch assigned that function and thus that syllable will change according to the key. The advantage of this system is that it trains the ear to recognize harmonic function, and recognizing harmonic function in a melody line assists in singing in tune. The disadvantages are that one must ascertain or be told what the tonic is before one can begin singing with movable do syllables, and also that accidentals are extremely challenging. To the latter point, because movable do expresses function, when the function of a note changes, the syllable must also be changed. Thus, unlike with the fixed do system, when, for example, the super tonic is flatted, the syllable must be changed from re to ra, whereas in fixed do both super tonic and flatted super tonic are called re in the key of C.
Whereas, at least conceptually, movable do is a functional system, fixed do, or so it has been claimed, is not a functional system. This is because fixed do syllables do not indicate function, but instead indicate pitch name. Fixed do syllables are equivalent to letter note names, but are easier to sing; however, I take issue with those who claim that fixed do precludes functions. People naturally perceive the tonic function in tonal music, regardless of syllables being used, or in fact even in the absence of any syllables at all. My students have no trouble locating the tonic note of a tonal melody, because it is the only pitch that satisfies their ear as a final or resting tone. That being so, it is but a small matter to affix a name to the pitch to which one has intuitively assigned a function. Once the note, let us say fa which is an F, has been established as the tonic, one can easily perceive and understand fa to be the tonic, do to be the dominant, ti to be the sub dominant, and so forth. In addition, one can also be sure of exactly what pitches are being heard, making the bridge from listening to sight reading easier. If anyone should doubt this, let him sing a major scale starting on fa, and then realize that fa already unmistakably sounds like nothing else but the tonic function. Singing a major scale beginning on other syllables will produce the same result. What is more, whether sharp or not, the note is sung with the same syllable. A major scale beginning on re would be re mi fa so la ti do re. The singer must adjust by ear the fa and do to be sharped. This in itself is a worthwhile ear training exercise, which, by the way, Dalcroze was extremely fond of, that the practitioner of movable do never encounters. To him, every key is the same. But we know this is not how the composers heard the keys, or else they would not have described some as bright, others dark or any number of other ways. Having different syllables for the different scales helps one hear each key differently, as they were meant to be heard.
Teaching functional harmony is important. The teacher using fixed do must not overlook teaching harmony because it is essential for understanding tonal music, and for singing in tune. Because the syllables in the fixed do system indicate pitch and not function, (though as we have seen function is perceived when using fixed do), the use of fixed do necessitates a separate designation for functions. Renowned teachers such as Nadia Boulanger used syllables for pitch and numbers for function. Using numbers for function and syllables for pitch emphasizes that pitch and function are two different concepts. A single musical tone has a definite pitch, but no function. A tone can only have function when it is perceived in the context of other tones. For example, fa could be tonic in f, dominant in b-flat, or mediant in d minor. The listener simply doesn’t know until other tones have placed that fa into a harmonic context. As long as the function of the note is unknown, it cannot be sung using movable do, but it can be sung using fixed do.
Using numbers to indicate function is consistent with the practice of numbering the scale degrees, with the tonic being ^1, the dominant being ^5, and so forth. It can be seen that ^1 in the fixed do system is the equivalent to do in the moveable do system. Because the syllables in the movable do system indicate function and not pitch, the use of movable do necessitates a separate designation for pitch. The musician using movable do does not know what the pitch name is unless letter names are also taught, just as the one using fixed do does not know what the function is unless numbers are also taught. So both systems have similar omissions if only syllables are taught. For this reason, it is necessary for the teacher of fixed do to teach syllables and numbers, and the teacher of movable do to teach syllables and letter names. Movable do syllables and numbers is redundant because both indicate function, and fixed do syllables and letter names are redundant because both indicate pitch.
Largely because each pitch is consistently given the same associated label, many who use fixed do often and consistently find that it develops in them a sense of absolute pitch. I have found this to be true for myself. During the school year when I am teaching with fixed do daily, I find myself starting songs with my voice on the correct pitch after reading the notation without much thought. I also find that I am less likely to drift flat over the course of a teaching session when I am mindful of the fixed do syllables. I believe this is how fixed do helps musicians sing more in tune. When relative pitch is relied on, as it must be with movable do, one has nothing to keep the pitch from drifting. But with fixed do, when the mind has secured pitches to a syllable, it retains the intonation of the pitch. Finally, it should be noted that the two systems do not go well together. Both have their attributes, and a teacher is can easily be justified choosing either, but should under no circumstance teach both to students concurrently. The cognitive advantages of associating notes with syllables is lost when the meaning of the syllables is contradictory from one system to the other.