The long s , also known as the medial s or initial s, is an archaic form of the lowercase letter . It replaced the single s, or one or both of the letters s in a "double s" sequence (e.g., "sinfulness" for "sinfulness" and "possess" or "possess" for "possess", but never *"possess"). The modern letterform is known as the "short", "terminal", or "round" s. In typography, it is known as a type of swash letter, commonly referred to as a "swash s". The long s is the basis of the first half of the grapheme of the German alphabet ligature letter , (eszett or scharfes s [sharp s]).
This list of rules for the long s is not exhaustive, and it applies only to books printed during the 17th and 18th centuries in English-speaking countries. Similar rules exist for other European languages.
A round s is always used at the end of a word ending with s: "his", "complains", "success"
However, long s is maintained in abbreviations such as "s." for "substantive" (substantive), and "Genes." for "Genesis" (Genesis)
Before an apostrophe (indicating an omitted letter), a round s is used: "us'd" and "clos'd"
Before and after an f, a round s is used: "offset", "satisfaction."
Before a breaking hyphen at the end of the line, a long s must be used: "Shafts-bury". (When hyphenation was not required, the word was spelled , with a round s.)
In the 17th century, the round s was used before k and b: "ask", "husband"; in the 18th century: "ask" and "husband".
Otherwise, long s is used: "song", "substitute".
In handwriting, these rules do not apply—the long s is usually confined to preceding a round s, either in the middle or at the end of a word—for example, "assure", "Blessings".
The long s was derived from the old Roman cursive medial s, . When the distinction between majuscule (uppercase) and minuscule (lowercase) letter forms became established, toward the end of the eighth century, it developed a more vertical form. During this period, it was occasionally used at the end of a word, a practice that quickly died but that was occasionally revived in Italian printing between about 1465 and 1480.