Σ ς,
σίγμα or σῖγμα (both accents are found in codices), τό, twentyfirst letter of the Etruscan abecedaria, [[ “IG” 14.2420] Refs], and probably of the oldest Gr. alphabets (corresponding to the twenty-first Hebrew letter shin ??, Phoenician Ω, [[ “Syria” 6.103), but eighteenth of the _Ionic dialect_ alphabet: as numeral σ = 200, but σ᾽ ] Refs] = 200,000: a semi-vowel, [[ 4th c.BC: Aristoteles Philosophus “Poetica” 1456b28, compare 5th-6th c.BC: Plato Philosophus “Theaetetus” 203b. ] Refs 5th c.BC+]
__A the oldest forms expressing this sound were Μ (which is however the old eighteenth letter, which see), also Σ and ς; compared to a twisted curl, [[ 5th c.BC: Euripides Tragicus “Fragmenta” 382.7, 4th c.BC: Theodectes Tragicus 6 ] Refs 5th c.BC+]; to a Scythian bow, [[ 5th c.BC: Agatho Tragicus 4 ] Refs 5th c.BC+]; after this, but yet early, it took the shape of a semicircle ??, whence [[ 4th c.BC: Aeschrio Lyricus (“fragment” 1) calls the new moon τὸ καλὸν οὐρανοῦ νέον σῖγμα: hence the orchestra is called τὸ τοῦ θεάτρου σῖγμα, 9th c.AD: Photius Legal icographus, etc., “Anecdota Graeca - Bekker” 286] Refs 4th c.BC+]: and Latin writers used sigma of a semicircular couch, [[ 1st c.AD: Martialis Epigrammaticus Latinus 10.48.6, etc.] Refs 1st c.AD+]; compare σιγμοειδής. The rare form ?? is used in the numbering of building-stones in [[ “Berl.Sitzb.” 1888.1234, 1242 (Pergam.)] Refs]. From final ς must be disting uished the character ς = 6, see ϝ ϝ (sixth letter).
__B the name σίγμα (σῖγμα) was usually indeclinable, τοῦ σῖγμα [[ 5th-6th c.BC: Plato Philosophus [same place], “Cra.” 402e, 427a, 2nd c.AD: Athenaeus Epigrammaticus 10.455c, 6th c.AD: Lydus, Joannes Laurentius Historicus “de Mensibus” 1.21 (variant{σίγματος}); τῷ σῖγμα 2nd c.AD: Galenus Medicus “de Usu Partium” 2.14, +others; τῶν σῖγμα 5th-6th c.BC: Plato Comicus 30; τὰ σίγμα τὰ ἐπὶ τῶν ἀσπίδων 5th-6th c.BC: Xenophon Historicus “Historia Graeca (Hellenica)” 4.4.10, compare 2nd c.AD(?): Helladius Epigrammaticus cited in 9th c.AD: Photius Legal icographus, etc. “Bibliotheca - Patrologia Graeca” p.532 5th c.BC: Bacchylides Lyricus; later declined, τοῦ σίγματος 12th c.AD: Eustathius Episcopus Thessalonicensis 1389.15; σίγμασιν [prev. author] 905.7. ] Refs 5th c.BC+]