[96]. Change of ai. ai is common in inscriptions: as, AIDILIS, PRAITOR. Toward the end of the republic the two elements of the diphthong had been partially assimilated to ae ([49]): as, aedīlis (Quint. 1, 7, 18). This is its pronunciation in the classical period. Between 130 and 100 B.C. ai is displaced by ae in public documents; but the old-fashioned ai was often retained in private inscriptions. Still later the two elements completely converged to ē. In provincial Latin ē is found as early as 200 B.C.: as, CESVLA for CAESVLLA; in Rome itself the pronunciation ‘Cēcilius’ for Caecilius, and ‘prētor’ for praetor was derided as boorish; but by 71 A.D. ae was verging toward ē even in the court language: the coins of Vespasian have IVDEA as well as IVDAEA. In the 3d and 4th century A.D. ē became the prevalent sound.
[97]. Change of au. The diphthong au, which was preserved in educated speech, was changed to ō in rustic and colloquial pronunciation (see the anecdote related by Suetonius, Vesp. 22): as, cōpō, innkeeper, for caupō; plōstrum for plaustrum (barge), cart: Clōdius for Claudius. Some of these gained literary currency: as, cōdex, book, caudex, block; fōcāle, neckcloth, faucēs, throat. The form sōdēs ([1572]) for si̭ audēs = sī audēs (Cic. O. 154) is a colloquialism.
[98]. Change of ei. ei as a genuine diphthong is common in old inscriptions: as, SEI; SEIVE; ADEITVR; DEIXERVNT; FEIDA. In classical Latin it has passed into ī: thus, sī, if; sīve, either; adītur, is approached; dīxērunt, they said; fīda, faithful. An intermediate stage between the old diphthong ei and the classical ī was a very close ([46]) ē: as, PLOIRVME ([465]) for plūrimī; IOVRE ([501], [507]) for iūrī. For the orthographical use of ei as a spelling for the long ī-sound, see [29].
[99]. Change of oi. The development of oi was parallel to that of ai. It first passed into oe: as, COIRAVERVNT and COERAVERVNT, they cared; OITILE, useful, and OETI, to use; LOIDOS and LOEDOS, play,—all in old Latin. In classical Latin it has further been changed in accented syllables to ū: as, cūrāvērunt, ūtile, ūtī, lūdus. But oe was retained in classical Latin (1.) when a secondary diphthong ([48]), the result of contraction ([120]), and (2.) in a few words like foedus, treaty, perhaps as an archaizing, legal term; foedus, ugly; poena, penalty, perhaps through the influence of Greek ποινή (in the verb pūnīre, to punish, the regular ū appears); proelium, skirmish; foetor, stench; and moenia, walls, perhaps because there was a word mūnia, services. The connection of nōn, not, with noenum ([455]; [1444]; [699]) is difficult because of the unusual development of oe to o, for which the Praenestine form CORAVERONT is the only parallel.
[100]. Change of ou. ou, found in inscriptions down to about 90 B.C., passed, in classical Latin, into ū: as, POVBLICOM, NOVNTIATA, IOVDEX; later pūblicum, public, nūntiāta, notified, iūdex, judge.
[101]. Change of eu. Primitive ([48]) eu appears in classical Latin only in the interjections eu, heu, ē̆heu, heus. Every other original eu had, even in old Latin, passed into ou and developed like the latter: as, *neumen (Greek νεῦμα) became first *noumen, then ([100]) nūmen. With the exceptions noted above, the diphthong eu, as it appears in Latin, is always of secondary origin ([48]), the result of the two vowels e and u meeting in composition: as, neu, neither, from nē-ve; neutiquam, from nē and utiquam ([124]).
[WEAKENING IN UNACCENTED SYLLABLES.]
[102]. The vowel of an unstressed (atonic) syllable is often weakened, changing its quantity or quality or both. This is especially the case in syllables immediately preceded by the chief accent (posttonic syllables). The following changes took place at an early period when Latin still possessed the old, recessive accent ([89]).
[WEAKENING OF SIMPLE VOWELS IN MEDIAL SYLLABLES.]
[103]. ([a.]) Atonic medial e before a single consonant was weakened (with the exceptions given under b.) to i: as, cólligō, collect, from legō; óbsideō, besiege, from sedeō; cértāminis, of the contest, from certāmen ([224]); flāminis, from flāmen ([470]). And so probably hic ([664]) arose from *hec or *hoc ([105, g]) when used as proclitic ([92]). Before the labials p, b, f, and m this weakened sound was intermediate between i and u ([28]), and both spellings occur: as, quadripēs and quadrupēs, four-footed; alimentum, nourishment; monumentum, monument. The choice of i or u was probably governed by the quality of the stressed vowel in the preceding syllable: viz., u after o and u, and i after a, e, and i. But such distinction is only imperfectly maintained in classical Latin.