95. Of the six original diphthongs #au#, #ou#, #eu#, and #ai#, #oi#,
#ei#, the only one which preserved its original sound in the classical
period is #au#. Of the rest only #ae# (for older #ai#) and, in a few
words, #oe# (for older #oi#) remained diphthongs; all the others had
become monophthongs.
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).