Stigma (letter) sigma form ς never occurs inside a word and is never used as a numeral either. Uppercase forms of stigma as a numeral (Ϛ) are rare in practice; when they occur, they can often be confused with uppercase forms of another numeral symbol, koppa (Ϟϟ), which stands for 90. Stigma is encoded in Unicode as "Greek letter stigma" U+03DA (Ϛ) and "Greek small letter stigma" U+03DB (ϛ). Stigma (letter) Stigma (ϛ) is a ligature of the Greek letters sigma (Σ) and tau (Τ), which was used in writing Greek between the Middle Ages and the 19th century. It is also
Archaic Greek alphabets , and was probably an intermediate sound during the phonetic change from the earlier plosive clusters towards the later sound, possibly an affricate similar to . In one attested document, the Arcadocypriot Greek of Mantineia used an innovative letter similar to И (), probably derived from a variant of san, to denote what was probably a -like sound in environments reflecting etymological Proto-Greek . In the highly-divergent Pamphylian Greek, the letter digamma () existed side by side with another distinctive form . It has been surmised that in this dialect the sound may have changed to labiodental in some environments.