Stigma (letter) 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 used as a numeral symbol for the number 6. In this unrelated function, it is a continuation of the old letter digamma (originally Ϝ, cursive form ), which had served as a numeral since antiquity and was conflated with the σ-τ ligature in the minuscule handwriting of the Middle Ages. The στ ligature (, ) was one of many ligature forms that came into widespread use as
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.