San (letter) instance of Greek letter
San (Ϻ) was an archaic letter of the Greek alphabet. Its shape was similar to modern M or Mu, or to a modern Greek Sigma (Σ) turned sideways, and it was used as an alternative to Sigma to denote the sound /s/. Unlike Sigma, whose position in the alphabet is between Rho and Tau, San appeared between Pi and Qoppa in alphabetic order. In addition to denoting this separate archaic character, the name San was also used as an alternative name to denote the standard letter Sigma.Historical use
Sigma and san
The existence of the two competing letters Sigma and San is traditionally believed to have been due to confusion during the adoption of the Greek alphabet from the Phoenician script, because Phoenician had more sibilant sounds than Greek had. According to one theory,: 25–27  the distribution of the sibilant letters in Greek is due to pair-wise confusion between the sounds and alphabet positions of the four Phoenician sibilant signs: Greek Sigma got its shape and alphabetic position from Phoenician Šin (), but its name and sound value from Phoenician Samekh. Conversely, Greek Xi (Ξ) got its shape and position from Samekh (), but its name and sound value from Šin. The same kind of pair-wise exchange happened between Phoenician Zayin and Tsade: Greek Zeta has the shape and position of Zayin () but the name and sound value of Tsade, and conversely Greek San has the approximate shape and position of Tsade () but may originally have had the sound value of Zayin, i.e. voiced [z]. However, since voiced [z] and voiceless [s] were not distinct phonemes in Greek, Sigma and San came to be used in essentially the same function. 
According to a different theory, "San" was indeed the original name of what is now known as Sigma, and as such presents a direct representation of the corresponding name "Shin" in that position. This name was only later also associated with the alternative local letter now known as "San", whose original name remains unknown. The modern name "Sigma", in turn, was a transparent Greek innovation that simply meant "hissing", based on a nominalization of a verb σίζω (sízō, from an earlier stem *sigj-, meaning 'to hiss').
Moreover, a modern re-interpretation of the sound values of the sibilants in Proto-Semitic, and thus in Phoenician, can account for the values of the Greek sibilants with less recourse to "confusion".  Most significant is the reconstruction of Šin as [s] and thus also the source of the sound value of Sigma; in turn, Samekh is reconstructed as the affricate [ts], which is a better match for the plosive-fricative cluster value [kʰs] of Xi.
Whereas in early abecedaria, Sigma and San are typically listed as two separate letters in their separate alphabetic positions, each Greek dialect tended to use either San or Sigma exclusively in practical writing. The use of San became a characteristic of the Doric dialects of Corinth and neighboring Sikyon, as well as Crete. San became largely obsolete by the second half of the fifth century BC, when it was generally replaced by Sigma, although in Crete it continued in use for about a century longer. In Sikyon, it was retained as a symbolic mark of the city used on coin inscriptions (just as the likewise archaic Qoppa was used by Corinth, and a special local form of Beta by Byzantium). 
San could be written with the outer stems either straight () or slanted outwards (), and either longer or of equal length with the inner strokes (). It was typically distinguished from the similar-looking Mu (Μ) by the fact that San tended to be symmetrical, whereas Mu had a longer left stem in its archaic forms (, , ).
Outside Greece, San was borrowed into the Old Italic alphabets (𐌑, transcribed as Ś). It initially retained its M-shape in the archaic Etruscan alphabet, but from the 6th century BC changing its aspect to a shape similar to that of the d-rune .
The name of "San" lived on as an alternative (dialectal or archaic) name for "Sigma" even at a time when the letter itself had everywhere been replaced with standard Sigma. Thus, Herodotus in the late 5th century reports that the same letter was called "San" by the Dorians but "Sigma" by the Ionians. Athenaeus in his Deipnosophistae (c.200 AD) quotes an epigram which contained the spelled-out name of the philosopher Thrasymachus, still using "San" as the name for Sigma:
type of example of manifestation of representation of