When you learn sight singing, it's typical to use different words to represent different musical pitches corresponding to different notes of the scale. Some teachers and students prefer to sing the notes using numbers: 1, 2, 3. Others use solfege syllables: Do, Re, Mi... Which system is preferable? Does it even make any difference. I'd like to show 3 reasons why using solfege syllables is a better method of learning sight singing.
Of course, it's indisputable that using numbers is a more intuitive system. The syllables are just nonsense, while numbers have a specific meaning in terms of degrees of the major scale. If all else were equal, numbers would be the hands-down winner. Unfortunately, other things are not equal. There are 3 critical disadvantages in using numbers.
The first, and least important, is that one of the numbers, seven, is a two-syllable word. That means that if you're singing numbers on the various notes, you'll either have to add an extra phantom syllable on a non-existent note each time the seventh note of the scale is used, or, more likely, to shorten the word to something like "sev". This is awkward, but it's not fatal.
A second reason to use solfege is the ambiguity of numbers. Numbers are used for so many things in music. They are used for rhythm when you're counting the beats in a measure: 1, 2, 3, 4. They are used for harmonic analysis, although often Roman numerals are used in this context. Ordinal numbers are used to describe the notes of a chord: the third, the seventh, etc. Some organizations, barbershop societies, for example, use numbers to describe degrees of volume: 1 is soft and 10 is loud. If we throw a counting system describing pitches into the mix, there will often be some uncertainty which usage of numbers is intended, rhythm, pitch, volume, or something else.
The third, and most crucial reason to use solfege syllables is that there is no number to represent the half-tones between scale notes. If you start saying things like "five sharp", "five plus", or "five and a half", you introduce the problem of extra syllables again. And there's no easy way to shorten such a phrase. In contrast, there are separate syllables to use for the non-scale notes. Usually it's just a difference in the vowel. If "La" is raised, it becomes "Li". If it's lowered it becomes "Le". This is a clean and decisive way of solving the problem of too many syllables. It's true that a system of number singing is easier for the beginner to understand and more intuitive. Perhaps in the beginning of sight singing instruction numbers could be used. But the serious disadvantages of the number singing method ought to lead to more and more use of solfege syllables as the student increases in skill. By the time reading of accidentals is reached, solfege should be used exclusively.