Stigma is a ligature

Greek ligatures

Greek ligatures are graphic combinations of the letters of the Greek alphabet that were used in medieval handwritten Greek and in early printing. Ligatures were used in the cursive writing style and very extensively in later minuscule writing. There were many dozens of conventional ligatures...

 of the Greek

Greek alphabet

The Greek alphabet is the script that has been used to write the Greek language since at least 730 BC . The alphabet in its classical and modern form consists of 24 letters ordered in sequence from alpha to omega...

 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

Greek numerals

Greek numerals are a system of representing numbers using letters of the Greek alphabet. They are also known by the names Ionian numerals, Milesian numerals , Alexandrian numerals, or alphabetic numerals...

 for the number 6. In this unrelated function, it is a continuation of the old letter digamma

Digamma

Digamma is an archaic letter of the Greek alphabet which originally stood for the sound /w/ and later remained in use only as a numeral symbol for the number "6"...

 (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.

History and use

The στ-ligature was one of many ligature forms that came into widespread use as part of the minuscule writing style of Greek from the 9th and 10th centuries onwards. It is based on the lunate form (Ϲ) of the letter sigma.

Together with many other ligatures it continued in use in printing Greek during the early modern era. Between the 18th and 19th centuries, the use of ligatures in print gradually diminished. The στ-ligature was among the last to go out of use, around the middle of the 19th century.

The name stigma (στίγμα) is originally a common Greek noun meaning "a mark, dot, puncture" or generally "a sign", from the verb στίζω ("to puncture"); the related but distinct word stigme (στιγμή) is the classical and post-classical word for "geometric point; punctuation mark." Stigma was co-opted as a name specifically for the στ-sign, evidently because of the acrophonic

Acrophony

Acrophony is the naming of letters of an alphabetic writing system so that a letter's name begins with the letter itself. For example, Greek letter names are acrophonic: the names of the letters α, β, γ, δ, are spelled with the respective letters: ....

 value of its initial st- as well as the analogy with the name of sigma.

The numeral symbol, originally quite unrelated to the στ-ligature, developed out of the letter Ϝ, which stood for the sound /w/ in early pre-classical forms of the Greek alphabet. This symbol became obsolete as a letter during the classical era, but remained in use as part of the Greek alphabet-based system of numerals, where its value of 6 corresponded to its original place in the alphabet. In its handwritten forms, its shape changed from through to or during the Hellenistic period and late antiquity. Originally called wau, it was called digamma

Digamma

Digamma is an archaic letter of the Greek alphabet which originally stood for the sound /w/ and later remained in use only as a numeral symbol for the number "6"...

in classical Greek and episemon during the Byzantine era. It was conflated with the στ-ligature owing to the accidental similarities of their shapes. The association between the numeral 6 and the letter sequence στ became so strong that today, in Greece, the letter sequence ΣΤʹ or στʹ is often used in lieu of ϛʹ itself to write the number 6.

In modern practice, the term stigma is often applied to the symbol ϛ both in its function as a ligature and as a numeral, whereas the term digamma is normally used for the ancient letter representing /w/, which appears in modern print as or (the form has a large number of close variants).

In modern typefaces, lowercase stigma is similar in appearance to final sigma

Sigma

Sigma is the eighteenth letter of the Greek alphabet, and carries the 'S' sound. In the system of Greek numerals it has a value of 200. When used at the end of a word, and the word is not all upper case, the final form is used, e.g...

 (ς), but the top loop tends to be larger, and extends farther to the right. It can normally be distinguished from final sigma in the context, because the combination στ never occurs at the end of a word, and conversely the final 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 confuseable with uppercase forms of another numeral symbol, koppa

Koppa

Koppa is an archaic letter-like numeral character of the Cyrillic writing system. Its form are derived from the Greek letter Koppa ....

 (Ϟϟ), which stands for 90.

Stigma is encoded in Unicode

Unicode

Unicode is a computing industry standard for the consistent encoding, representation and handling of text expressed in most of the world's writing systems...

as "Greek letter stigma" U+03DA and "Greek small letter stigma" U+03DB .