The diphthongs ae, oe were formerly reduced to ę and ẹ ; au held out longer before shrinking to o ??? .
But one must distinguish on the one hand the place that vowels and consonants originally occupied in the word and, on the other hand, consider the position of the word in the sentence, distinguishing the treatment of tonic vowels (in the word or sentence). Furthermore, it is necessary to distinguish free vowels or open syllables, i.e. those found at the end of a syllable, p. ex. the a of ma – re or pa – trem, from the implied vowels or in a closed syllable, which are followed by a consonant that closes the syllable, p. ex. the a of pas – tum or par – tem.
In Gaul, perhaps due to Celtic influence, the Latin ū already took the sound of ü in ancient times (i.e. it passed from labiovelar to labiopalatal): p. ex. m ū rum mur [pron. mür ], duratum durü [pron. düré ].
Unstressed vowels are strongly prone to weakening. Less than the others the initial protons, which, uttered with particular clarity, have generally been kept in French, both with the original timbre (mari da mar ī tum, filer da f ī l ā re), and with an alteration that can be unconditional (duré [ düré ??? ] from duratum, couronne from coronam), but which is often due to the influence of neighboring sounds or to the fact that it is a closed syllable (cheval [ è ə val ] from caballum, cf. charbonfrom carbonem, venir [ v ə nir ] da venīre, ma merci da mercēdem).
According to HOMOSOCIETY, the final vowels are generally fallen since prehistoric times (nef from navem, hier from her ī, Loss [ s ] to lose, plein from plenum), except l ‘ in the final which was maintained for longer in pronunciation as and semimuta [ə], and still survives today in writing (mule da m ū la). However, after several consonant groups, all the original final vowels have been preserved in the form of a and semimuta, said support (doors da portam, chèvre da capram, fièvre da febrem, enfle da inflo, nôtre da nostrum) and in particular the Latin sdruccioli words (lièvre da lep ŏ rem, semble da sim ĭ lo, frêne da frax ĭ num). The penultimate unstressed vowels have fallen into different ages, sometimes very ancient (œil da oc [ u ] lum: oclum is already found in vulgar Latin, vert da vir [ ĭ ] dem, chaume da cal [ ă ] mum).
The semi-silent e, of whatever origin, had a precarious fate. Before a vowel it disappeared in ancient times, both in the final (the so-called elision: bell [ e ] amie), and elsewhere (armure from armëure, lat. Armature ; mûr from mëur, lat. Maturum, etc.). Followed by a consonant and preceded by a vowel, it disappears in ancient times: enrou (e) ment, l’anné (e) dernière. Between two consonants it has been eliminated by modern pronunciation (acheter, pron. in is tea), except in cases in which they would meet at three consonants (justement, pron. ž üst ə mã ; speak, pron. parl ə ré) or, absolute initial, two (Petit ! Demain!).
Tonic vowels have much greater resistance. In the closed syllable they have never diphthongs in the literary language (mil da mille, fer da ferrum, char da carrum, porte da portam); ẹ opened in ę, ọ closed in u (spelling ou): mettre da m ĭ ttere, courda c ü rtem. In open syllable i and u they are conserved (nid from n ī dum, mur [ie, as we said, mür ] from m ū rum); ẹ becomes ei, from which later oi: from me one has mei, moi (from which then, the spelling remaining unchanged, móe, mw ę, mwa), o, with a different evolution, becomes e (clay becomes craie); ę becomes ie, maintained to this day (miel); ọ becomes ou and then œ (fl ü rem gives flour and then fleur, now pronounced fl œr); or??? becomes uo and then ue, œ (cuor, cuer, c œur). Finally, ‘ a tonic has become free and (lat. Sea, fr. Wed) perhaps due to a prehistoric diphthongization. Thanks to these different evolutions, French ended up having a rich and original vowel system, made up of three complete ranges (the palatal range ę, ẹ, i), the labio-velar range (or ???, ọ, u), the labio-palatal range (œ [open and closed], ü).
The proximity of m, n, j affects the treatment of vowels (the j, voiced palatal spirant, is the continuator of the Latin j [ i with consonant value], as majus [ maius ], and of ĕ and ĭ which under certain conditions they are consonantized, eg. f ī l ĭ a, v ī nea trisyllables, become * filja, * vinja bisillabi). Thus, p. e.g., á + j gives aj, ej, ę (maium – mai), j + á + j gives * jaj and then i (iacet – gît), ó ??? + j gives * uei and then ui (c ŏ rium – cuir), o ??? + j gives oi (rasorium – satinir). In contact with m or n the vowel tends to modify the timbre, or to become nasalized, sometimes the two effects are combined. Eg. pl ē nam gives pleine (while habere gives avoir); grandem gives grand [pron. grã ]; vinum gives vin [pron. v æ ], that is, the i is not only nasalized, but opened in e. This shift gave rise to an original system of nasal vowels.
