Modern western musical notation began in medieval Europe, aimed at bringing ecclesiastical uniformity to the Catholic church, and by the mid 9th century European monasteries had begun to develop notation as a mnemonic device for Gregorian chant.
A century later, Italian Benedictine monk Guido d’Arezzo (991-1033) designed the standard musical stave, and taught solmization using syllables based on the first stanza of a hymn to Saint John The Baptist, written by Paulus Diaconus in the 8th century.
It reads: Ut queant laxis resonare fibris, Mira gestorum, famuli tuorum, Solve polluti labii reatum, Sancte Iohannes. This is paraphrased as: Do let our voices resonate most purely, miracles telling, far greater than many; so let our tongues be lavish in your praises, Saint John the Baptist.
In the 17th century, Ut was changed to Do, which represents the Latin word Dominus, meaning Lord and Si (from Sancte Iohannes) was added to complete the diatonic scale. However, Anglophone countries changed Si to Ti in the nineteenth century so that each syllable would begin with a different letter.