The Semitic languages characteristically keep several s sounds apart and treat them as different consonants, and the twenty-two letter alphabet reflects that with three separate signs: semkath, sadhe and shin. Getting them straight is one of the standing difficulties of comparative work in this family, because the correspondences between languages are not simple.
Within Syriac the position is clear enough. The three letters are distinct in spelling, distinct in the classical reading, and they belong to different roots. A reader has to know which letter a word takes, in the way an English reader has to know which of several spellings a word takes, and the alphabet gives no help.
Across languages the position is harder. When Syriac letters are set beside their Arabic counterparts by place in the abjad order, semkath at sixty answers to the Arabic letter at sixty and shin at three hundred to the Arabic letter at three hundred, and the match of place does not track the match of sound: the historical development of the sibilants in Arabic and in Aramaic did not run in parallel. The atlas records those pairings on the order alone and says so, rather than implying a phonetic identity that is not there.
Where a modern language written in Syriac letters needs sibilants the classical alphabet has no sign for, as Turoyo does, they are marked with added dots rather than given new letters.