As an initial and medial it has a hard and sharp sound (which is softened, however, between two vowels), and is therefore joined only with the tenues (c, p, t; cf., on the contrary, the Gr. σβέννυμι ); and, as a medial, often written double after long vowels: caussa, cassus, divissiones (these forms, used by Cicero and Vergil, were already uncommon in Quintilian's time, Quint. 1, 7, 20; v. Corss. Ausspr. 1, 283 sq.).
As a final it had a weakened sound, and therefore not only admitted the medial b before it (plebs, urbs, abs; Arabs, chalybs, etc.; v. the letter B), but often entirely disappeared. So in the ante-class. poets down to the early years of Cicero (and also in his own poem, entitled Aratus, written in his youth), before words beginning with a consonant, to avoid position: Ratu' Romulus, Fulviu' Nobilior, gravi' Terra, est sati' bella, Hyperioni' cursum, Virgine' nam sibi, etc.; cf. Cic. Or. 48, 161; Quint. 9, 4, 38; and v. Freund, in Jahn's Neue Jahrb. 1835, XIII. p. 25 sq.; less freq. before words beginning with a vowel, in which case, to avoid a hiatus, the vowel before s was also elided; vas' argenteis (for vasis argenteis) and palm' et crinibus (for palmis et crinibus); v. Cic. Or. 45, 153. So, too, in the fourth Epitaph of the Scipios (Inscr. Orell. 553), L. CORNELIO L. F. instead of CORNELIOS (cf. a similar elision of the M under that letter). Final s is also elided, and the preceding vowel either dropped with it or weakened, in the forms sat from satis, mage from magis; in the neutr. forms of adjectives of the third declension, acre, agreste, facile (v. the letter E); in the collat. forms of the sec. pers. sing. pass., fatere, fateare, fatebare, etc.; in the gen. sing. of the first, second, and fifth declensions, and in the nom. plur. of the first and second declensions (aurai for aura-is, analog. to regis, etc.). Lastly, s disappears in the (mostly familiar) collat. forms abin', scin', viden', satin', from abisne, scisne, videsne, satisne, etc.
As an etymological initial aspirate, s appears in many words whose Greek equivalents begin with a vowel: sal, semi-, serpo, sex, super, sus, corresp. to ἅλς, ἡμι-, ἕρπω, ἕξ, ὑπέρ, U/=ς, etc.; si (archaic sei), sero, Segesta, corresp. to εἰ, ἘΡΩ (whence εἴρω ), Ἔγεστα. Less freq. in radical words beginning with a consonant: sculpo corresp. to γλύφω, and the derivatives scruta, from γρύτη, and scrupedae, from κρούπεζα. To soften the termination, s appears in abs = ab, and ex corresp. to ἐκ. —Very freq., on the contrary, an initial s appears in cognate forms in other languages, where corresp. Latin words have lost the s: Lat. fallo, Gr. σφάλλω ; fungus, Gr. σφόγγος ; fides, Gr. σφίδη (comp. also nix with Engl. snow, nurus with old Germ. snur, daughterin-law); cf. also cutis and scutum; cauda and root sku-, in Goth. skauts, etc.; casa and Gr. σκιά, σκηνή ; cerno and Gr. κρίνω for σκίρνω, σκώρ, σκωρία ; calumnia and σκάλλω ; gradior and root scra-, Germ. schreiten; parco and σπαρνός ; penuria and σπάνις ; pando and σπάω ; tego and στέγω ; tono and στόνος ; taurus and Sanscr. sthūras, Germ. Stier al.; v. Corss. Ausspr. 1, p. 277 sqq.—In the middle of a word s is dropped in at from ast.
S is interchanged, Most freq. with r; in partic., an original s, between two vowels, becomes r; v. Varr. L. L. 7, § 26 Müll.; so foederum for foedesum, plurima for plusima, meliorem for meliosem, Lares for Lases, etc.; cf. eram and sum, quaero and quaeso, nasus and naris. Appius Claudius, the censor, is said to have introduced r into the names Furius, Valerius, etc., in place of s, B.C. 312 (v. the letter R, II.).
With d: Claudius, from the Sabine Clausus; and, on the other hand, rosa, corresp. to the Gr. ῥόδον ; cf. Schneid. Gram. 1, p. 259.
With t: tensus and tentus, resina corresp. to ῥητίνη ; and, on the contrary, aggrettus for aggressus; mertare, pultare, for mersare, pulsare (perh. also assentor for assensor).
With x; v. that letter.
S is assimilated before f in the compounds of dis: differo, difficilis, diffluo, etc.; v. 3. dis.
On the other hand, it arises by assimilation from d, in assum, assumo, cessi, for adsum, adsumo, ced-si; from t in fassus, from fateor; from b in jussi, from jubeo; from m in pressi, from premo; from r in gessi, from gero; and dossuarius, from dorsum.
As an abbreviation, S denotes sacrum, semis, sibi, suis, etc.; S. AS. D., sub asciā dedicavit; S. C., senatusconsultum; perh. also, sententia collegii (Inscr. Orell. 2385); S. P., sua pecunia; S. P. Q. R., Senatus Populusque Romanus, etc.