Semkath is the fifteenth letter of the Syriac abjad, with the numeral value sixty. It writes s, a voiceless alveolar sibilant, and it joins on both sides.
A separate final semkath is recorded as a sign of its own in the modern character standard. It is a positional variant of the same letter rather than an addition to the alphabet, which stays at twenty-two letters, and it is encoded separately because editions of manuscripts need to be able to show it.
Its name is the inherited word for a prop or a support, the sense that also stands behind Hebrew samekh at the same fifteenth place in the sequence.
Semkath is one of three sibilants the classical alphabet keeps apart. Sadhe, the eighteenth letter, writes the emphatic s, and shin, the twenty-first, writes the sh of English ship. All three distinctions were live in the language when the sequence was fixed, and classical Syriac maintains them in spelling throughout. A Syriac word is spelled with one of the three and not with another, and the choice is not free: it belongs to the root.
The three letters also fall at three widely separated places in the alphabet, at fifteen, eighteen and twenty-one, with numeral values sixty, ninety and three hundred, so nothing about the sequence groups them together.