In this system, 1 is always the root or origin, but the scale being represented may be major, minor, or any of the diatonic mode.
Accidentals (sharps and flats outside the key signature) are noted with a + or - when the numbers are written, but are often skipped when they are spoken or sung.
In some pedagogies involving numerical sight-singing notation students are not taught to modify vowels to represent sharp or flat notes.
A drawback often pointed out is that numerical numbers are not always "singable," for example, scale degree 7 (ti, in solfege) contains vowels that are hard to tune.
Nor is it the same as "count singing", a technique popularized by Robert Shaw in which the numbers sung represent the rhythms of a piece in accordance with the beat of a measure.