The eighteenth letter of the
Greek alphabet, capital form Σ and lower-case σ . At the end of a word a different lower-case form ς is used. The HTML codes for these are Σ and σ and ς
It derives from a Semitic letter such as Hebrew ש sin, though the name sigma is perhaps influenced by that of a different sibilant, ס samekh. It occupies the position of sin. It gave rise to the Roman letter S.
The basic sound was S in Ancient Greek, and continues to be S in Modern Greek. The only significant variation was before voiced consonants such as B, G, L, M, N: so Lesbos and Smyrna were pronounced Lezbos, Zmyrna. This also continues in Modern Greek. It was also voiced before D but the combination ZD was usually written as ζ zeta.
Where most dialects had it doubled, SS, the Attic dialect of Athens used ττ TT. This probably derived from an original TT that changed in all other dialects, and would have had an intermediate value like TS at some point before the classical period, but there is no evidence that Attic σσ was anything other than a normal doubled SS in pronunciation.
Note that the group GM was pronounced ngm as in hangman in Ancient Greek (see agma), so the name was actually said singma.
An old variant form of the capital letter was C, which gave rise to the Cyrillic letter of that shape, representing S, as in CCCP for the USSR. This must not be confused with the unrelated Roman letter C, a derivative of Γ gamma.
Ancient Greek S came from Proto-Indo-European S in some positions, such as the end of a word (noun ending -ος -os) and before consonants (αστηρ astêr = English star = Latin stella from ster-la). However, S before a vowel changed to H long before the classical period (even before Mycenaean), and disappeared between vowels. So hex-, herpet-, hemi- are the Greek elements for 'six, serpent, half' (Latin semi-).
The old genitive ending was -osyo (cf. Sanskrit -asya). In Homeric Greek this has changed to -oio, and it later went to -oo then classical -ou.
Instances of sigma before vowels in classical Greek derive from other sources, such as tw, tj, thj (earlier dhj), and sw, or borrowings: the second person pronoun συ su corresponds to Latin tu, English thou. The word sêma 'sign' is related to Sanskrit dhyâman 'thought'. The word selênê 'moon' comes from earlier swelannâ. (Actually, sw also lost its s normally, but the origin of selênê was confirmed to me by Miguel Carrasquer Vidal, the linguist I most respect.)
In the Laconian dialect sigma was used where other dialects have theta. It is possible that this was pronounced not S but Th as in thin. The letter theta later changed to this value throughout Greek, so perhaps Laconian was the first to use it.