([3.]) The combinations of unsyllabic ([83]) ṷ with the vowel u and of unsyllabic i̭ with the vowel i were avoided in classical Latin; see [52].
([4.]) In composition, unsyllabic ([82]) i̭ after a consonant became syllabic in quoniam, since, for quomi̭am ([164, 5]), and etiam, also, for eti̭am (both compounds with iam).
[154]. In early Latin s between two vowels was voiced ([75]), and in the fourth century B.C. this voiced s changed into r. According to Cicero (Fam. 9, 21, 2) L. Papīrius Crassus, consul in 336 B.C., changed his family name Papīsius to Papīrius. Old inscriptions show frequently s for r: as, ASA, altar, AVSELII. This change of intervocalic s to r plays an important part in declension, conjugation, and derivation: as,
Nominative iūs, right, genitive iūris; spērō, I hope, derived from spēs; nefārius, wicked, from nefās; gerō, I carry, from a stem ges- which appears in ges-sī, ges-tus ([953]); erō, I shall be, from the stem es- in esse; the subjunctive ending -sem in es-sem appears as -rem after vowels: as, stārem; the infinitive ending ([894], [895]) -se in es-se appears as -re after vowels: as, legere, for *legese, to read, stāre, for *stāse, to stand. Where all oblique cases show -r- and only the nominative singular -s, the latter is sometimes changed to -r by analogy: as, arbor, tree, honor, honour, for original arbōs, honōs, by analogy to the oblique cases arboris, arborī, honōris, honōrī, etc. ([487], [488]). The final -s of the prefix dis- follows this rule: as, dir-imō, I take apart, for *dis-emō; but an initial s- of the second member of a compound remains unchanged: as, dē-sinō, I stop.
[155]. Wherever intervocalic s is found in classical Latin it is not original, but the result (a.) of earlier -ns-: as, formōsus, handsome, for formōnsus ([63]); ([b.]) of earlier -ss- ([170, 7]): as, ūsus for *ūssus, use ([159]); causa, thing, for caussa (Quint. 1, 7, 20); or (c.) it occurs in borrowed words like asinus, ass. (d.) There are a few words in which an r in a neighbouring syllable seems to have prevented the change: as miser, miserable ([173]).
[156]. Before the o described in [142] qu changed to c: as, incola, inhabitant, for *inquola, from *inquela; the stem quel- appears in in-quil-īnus, lodger.
[157]. As v before u ([107, c]), so qu was not tolerated before u, but changed to c.
Hence when, about the beginning of our era, the o of quom, when, sequontur, they followed, changed to u ([107, c]), they became cum, secuntur; thus equos but ecus, horse ([452]); reliquom but RELICVM, the rest; loquor, I speak, but locūtus ([978]). Much later, in the second century of our era, the grammarians restored the qu before u by analogy to those forms in the paradigm in which qu came before other vowels: as, sequuntur for secuntur by analogy to sequor, sequeris, sequitur, sequimur, sequimini, etc.; equus, equum, for ecus, ecum, by analogy to equī, equō, eque, equōrum, equīs, equōs.
[158]. qu before consonants or when final changed to c: as, relictus from the stem liqu-, leave (present, linquō, [938]); ac, and, for *atc, by apocope from atque; nec, nor, by apocope from neque. See also *torctus ([170, 3]), quīnctus ([170, 4]).
[159]. When in the process of early word formation a t was followed by another t, the combination tt, unless followed by r, changed to ss: as, obsessus, besieged, sat upon, for *obsettus, from *obsed-tus (cf. sedeō). After long vowels, nasals, and liquids this double ss was simplified to s ([170, 7]): as, ūsus from *ūt-tus, used (cf. ūtor); scānsus, climbed, from *scant-tus for *scandtus (cf. scandō).