Stigma (uppercase Ϛ, lowercase ϛ) is a ligature of the Greek letters sigma and tau, sometimes used in modern times to represent the Greek numeral 6. However, today the letters στʹ (ΣΤʹ, if capitalised) are more widely used to represent the number 6 or the ordinal 6th.
It is encoded in Unicode as "Greek letter stigma" U+03DA (Ϛ) and "Greek small letter stigma" U+03DB (ϛ). Small stigma is similar in appearance to final sigma (ς), but the top loop tends to be larger, and extends farther to the right. 'Stigma' is also a name for the cursive graphic form of the letter digamma when used as the Greek numeral 6. This name is a modern misunderstanding. The ligature of στ did not occur in ancient times and only dates to later medieval manuscripts. In Sophocles' Lexicon of Byzantine Greek, which covers late antique Greek up to 1000 AD, there is no mention of "stigma" as either a ligature or a number.
