S is the nineteenth (19th) letter

Letter (alphabet)

A letter is a grapheme in an alphabetic system of writing, such as the Greek alphabet and its descendants. Letters compose phonemes and each phoneme represents a phone in the spoken form of the language....

 in the ISO basic Latin alphabet.


Semitic

Semitic languages

The Semitic languages are a group of related languages whose living representatives are spoken by more than 270 million people across much of the Middle East, North Africa and the Horn of Africa...

 Šîn

Shin (letter)

Shin literally means "Sharp" ; It is the twenty-first letter in many Semitic abjads, including Phoenician , Aramaic/Hebrew , and Arabic ....

 ("teeth") represented a voiceless postalveolar fricative

Voiceless postalveolar fricative

The voiceless palato-alveolar fricative or voiceless domed postalveolar fricative is a type of consonantal sound, used in many spoken languages, including English...

 /ʃ/ (as in ship). Greek

Greek language

Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 did not have this sound, so the Greek sigma (Σ) came to represent /s/. In Etruscan

Etruscan language

The Etruscan language was spoken and written by the Etruscan civilization, in what is present-day Italy, in the ancient region of Etruria and in parts of Lombardy, Veneto, and Emilia-Romagna...

 and Latin

Latin

Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

, the /s/ value was maintained, and only in modern languages has the letter been used to represent other sounds.

The minuscule form of s was ſ, called the long s

Long s

The long, medial or descending s is a form of the minuscule letter s formerly used where s occurred in the middle or at the beginning of a word, for example "ſinfulneſs" . The modern letterform was called the terminal, round, or short s.-History:The long s is derived from the old Roman cursive...

, up to the fifteenth century or so, and the form 'S' was used then only as upper case, just like 'G' and 'A' were only upper case.