'Ti' (solfège) originates from the tonic‑sol‑fa/solfège tradition; it is a modern English variant of the syllable 'si' introduced so that each syllable began with a different letter.

'Ti' changed from the earlier 'si' (used in several European languages). The solmization syllables themselves go back to the Latin hymn 'Ut queant laxis' (Guido of Arezzo), where the sequence began as 'ut, re, mi, fa, sol, la'; 'si' was later formed (and 'si' became 'ti' in some English‑language pedagogies in the 19th century, e.g. Curwen/Glover) and eventually the English form 'ti' became common.

Initially it denoted the syllable 'si' or the seventh tone in older continental systems; over time the English form 'ti' came to be used to name the seventh scale degree (the leading tone) distinctly.