In music, solfège (UK: /ˈsɒlfɛdʒ/, US: /sɒlˈfɛʒ/; French: [sɔlfɛʒ]) or solfeggio (/sɒlˈfɛdʒioʊ/; Italian: [solˈfeddʒo]), also called sol-fa, solfa, solfeo, among many names, is a music education method used to teach pitch and sight singing of Western music. Solfège is a form of solmization, and though the two terms are sometimes used interchangeably, the systems used in other music cultures such as swara, durar mufaṣṣalāt and Jianpu are discussed in their respective articles.

Syllables are assigned to the notes of the scale and enable the musician to audiate, or mentally hear, the pitches of a piece of music which he or she is seeing for the first time and then to sing them aloud. Through the Renaissance (and much later in some shapenotepublications) various interlocking 4, 5 and 6-note systems were employed to cover the octave. The tonic sol-fa method popularized the seven syllables commonly used in English-speaking countries: do (or doh in tonic sol-fa), re, mi, fa, so(l), la, and si (or ti), see below).

There are two current ways of applying solfège: 1) fixed do, where the syllables are always tied to specific pitches (e.g. “do” is always “C-natural”) and 2) movable do, where the syllables are assigned to scale degrees (“do” is always the first degree of the major scale).

https://en.wikipedia.org/wiki/Solf%C3%A8ge