S, the nineteenth letter of the English Alphabet, is a sibilant articulation, and numbered among the semi-vowels. It represents the hissing made by driving the breath between the end of the tongue and the roof of the mouth, just above the upper teeth. It has two uses; one to express a mere hissing, as in Sabbath, sack, sin, this, thus; the other a vocal hissing, precisely like that of z, as in muse, wise, pronounced muze, wize. It generally has its hissing sound at the beginning of all proper English words, but in the middle and end of words, its sound is to be known only by usage. In a few words it is silent, as in isle and viscount.

In abbreviations, s stands for societas, society, or socius, fellow; as F.R.S. fellow of the Royal Society. In medical prescriptions, s adjective signifies secundem artem, according to the rules of art.

In the notes of the ancients, s stands for Sextus; SP. for Spurius; s C. for senatus consultum; s P.Q.R. for senatus populusque Romanus; s S.S. for stratum super stratum, one layer above another alternately; s V.B.E.E.Q.V. for sivales, bene est, ego quoque valeo.

As a numeral, s denoted seven. In the Italian music, s signifies solo. In books of navigation and in common usage, s stands for south; s E. for south-east; s W. for south-west; s S.E. for south south-east; s S.W. for south south-west, etc.