Teth is the ninth letter of the Syriac abjad, with the numeral value nine. It writes an emphatic t, one of the consonants that Semitic languages set apart from their plain counterparts by a secondary articulation further back in the mouth. In the classical reading the contrast between teth and taw is a real one, and the two letters are not interchangeable in spelling.
Teth joins on both sides and takes initial, medial, final and isolated shapes. Its name is the inherited Northwest Semitic word usually glossed as a serpent, a reconstruction of a sign in the older linear alphabet rather than a description of the Syriac form.
A marked variant of teth belongs to Garshuni, the writing of Arabic in Syriac letters, where it stands for an Arabic sound the alphabet has no letter for. Like the marked Garshuni gamal, it is a convention of that use and not a twenty-third letter of the Syriac alphabet. The abjad remains twenty-two letters however many marked signs a manuscript happens to carry.
Estrangela, Serto and Madnhaya each draw teth in their own way, and this entry is for the letter that all three write. Where a claim holds for only one hand, the atlas says which.
Teth carries the numeral value nine and is therefore the last of the units, so a scribe writing a figure in Syriac letters passes from teth to yudh exactly where the count passes from nine to ten. Nothing about the letter marks that boundary; it falls there because the inherited order puts it there.