A European system often used to teach singing, where a syllable is associated with each degree or note of the standard Western diatonic scale. The idea is credited to Guido d’Arezzo in the 11th century, who was trying to give his students a reference for singing based on a familiar melody.
There are two systems in use. The oldest is based on six syllables from a Latin hymn/Gregorian chant for St. John the Baptist’s Day, “Ut Queant Laxis,” composed by Paulus Diaconus (Paul the Deacon) in the 8th century. Each phrase of the hymn begins on a successive ascending note of the scale. The first syllables of each phrase were ut, re, mi, fa, sol, and la, which covered the first six notes of the diatonic scale. The seventh scale degree was covered using the initials of St. John found in the last two words of the last line, “Sancte Iohannes” (si).
More common in the United States is the 19th-century system using the syllables do, re, mi, fa, sol, la, and ti. The first syllable was changed because musical theorists felt “do” was a more open, sustainable vocal sound than “ut.” (Some credit the change to Giovanni Battista Doni, who is said to have used the first two letters of his last name to represent the tonic note.) There are two variations on this system, one where “do” is fixed on the note C (“fixed do“), and the other where “do” is moved to indicate the tonic of the key being sung (“movable do” or “tonic sol–fa“).