From BiblePortal Wikipedia

Hastings' Dictionary of the Bible [1]

TETH . The ninth letter of the Heb. alphabet, and as such employed in the 119th Psalm to designate the 9th part, each verse of which begins with this letter.

Holman Bible Dictionary [2]

 Psalm 119:65-72

International Standard Bible Encyclopedia [3]

teth ט : The 9th letter of the Hebrew alphabet; transliterated in this Encyclopedia as (a more intense t ). It came also to be used for the number 9; and with wāw ו for 15, with zāyin ז for 16 (i.e. 9 plus 6,9 plus 7) to avoid forming regular series with the abbreviation for Yahweh. For name, etc., see Alphabet .