The long or medial s (ſ) is a form of the minuscule letter s that was formerly used when the s occurred within or at the beginning of the word, for example ſinfulneſs ("sinfulness"). The modern letterform was called the terminal or short s.
The medial s was subject to confusion with the minuscule f, sometimes even having an f-like nub appended to its middle, in various derivatives of Roman typeface and in blackletter. There was no nub in its typeform in italics, which also gave the stroke a descender which curled to the left, something which could not be given to the other typeforms mentioned without kerning.
The nub derived from its form in the blackletter style of penmanship. What looks like one stroke to us was actually a wedge pointing downward, whose widest part was at that height, capped by a second stroke forming a curled ascender to the right. Those styles of writing and their derivatives in type design not only had a cross-bar at height of the nub in the form of the letters f and t, but also k, and had a similar nub at the same height for the letters b, h and l. In Roman type, these disappeared except for the one on the medial s.
The medial s was once used in ligatures in various languages. Three examples were for si, ss, and st, besides the German ß.
The practice of using the long s later died out in Roman and Italic type-faces well before the end of the 19th century. In most countries the ligatures vanished as well. Yet some French typefaces still have a ligature for st, which uses the other, or terminal s, with a little curl connecting the two letters on the top in a manner similar to that of the former ligature using the long s. The prestigious Editions Pleiades use this in italics.
It survived longer in the German blackletter typefaces; even the modern German letter ß (ess-tsett) is a ligature representing either ſz or ſs (see ß for the dispute). One may note that Greek also features a normal sigma 'σ ' and a special terminal form 'ς', which may have supported the idea of distinct 's' forms; in Renaissance Europe a significant percentage of the literate class would have been familiar with Greek.
The medial s survives in an elongated form as the integral symbol used in calculus ∫: Gottfried Wilhelm von Leibniz based the character on the latin word summa (sum), which he wrote ſumma. In linguistics, the same symbol is used in the International Phonetic Alphabet, in which it represents the first sound in the English word shun.
The medial s is represented in Unicode by the sign U+017F, and may be represented in HTML as
The confusion between the medial s and f has been the subject of some intentional humour, much of it involving phrases like "sucking pig", and forms the basis of Benny Hill's song "Fad-Eyed Fal" (i.e., Sad-Eyed Sal). It has been proposed as a possible etymology of the hacker jargon term cruft.