Shin is the twenty-first letter of the Syriac abjad, with the numeral value three hundred. It writes a voiceless postalveolar sibilant, the sh of English ship, and it is kept distinct from semkath throughout the classical language. It joins on both sides.
Its name is the inherited Northwest Semitic word for a tooth. The name and the twenty-first place are the same in Hebrew, and shin is one of the fixed points a reader uses to map one Aramaic alphabet onto another.
In Turoyo, a modern Aramaic language written in Syriac letters, three dots placed above or below a letter mark sounds the classical alphabet has no sign for, and the sibilants are among the letters that most often carry them. That is the same habit of extension that produced the Garshuni signs and the marked letters of Sogdian and Persian: the alphabet is left alone and marks are added to it.
Shin sits third from the end of the sequence, with the numeral value three hundred, in the run of four letters that carry the hundreds. Its place, its value and its name all come from the inherited Aramaic order, unchanged.
The sound shin writes is one that the Semitic languages did not develop uniformly, and the letter is a standing difficulty in comparative work, because a Syriac word spelled with shin and an Arabic word that looks like its cognate may not carry the matching letter. Within Syriac itself there is no difficulty at all: shin is distinct from semkath and from sadhe, the spelling of a root fixes which of the three it takes, and the classical reading keeps all three apart.