In all diphthongs the transition from one vowel to the other is gradual. A diphthong is, therefore, not formed simply by pronouncing two vowels in succession, but the vocal organs pass through all the intermediate positions and consequently the sound is constantly changing.
[48]. In their origin diphthongs are of two kinds: (a.) primitive diphthongs: as in foedus, treaty; aurum, gold; or (b.) secondary diphthongs, the result of vowels meeting in formation, composition, or inflection: see [120].
[49]. The diphthongs which occur in classical Latin are au, ae, oe, and the rare ui and eu.
au sounded like ou in house. ae had the sound of short Latin a rapidly combined with the sound of e in English men. But it is the common practice now to give to ae the sound of ay or ai in ay, aisle, although the difference between Latin ae and the earlier ai from which it descended is thus obliterated. oe had the sound of short Latin o rapidly followed by the sound of e in English men. But it is now customary not to distinguish between Latin oe and oi, and to give to both the sound of oi in boil. ui is pronounced by combining Latin short u and i ([40], [41]) with the stress on the i like French oui; eu by combining Latin short e and u with stress on the u.
[50]. Besides these, the following diphthongs occur in the older inscriptions: ai pronounced as ai in aisle; ei as ei in eight; oi as oi in boil; and ou which sounded very much like the final o in no, go, which is really a diphthong (see [39]).
[CONSONANTS.]
[51]. Consonants are formed by stopping the breath somewhere in the cavity of the mouth or by squeezing it through a narrow channel or aperture.
[52]. Semivowels. There is no sharp line of demarcation between consonants and vowels. Some vowels in unsyllabic function ([82], [83]) notably i (i̭) and u (ṷ) (corresponding to English y and w), though usually classed as consonants, are so closely related to the vowels that they are termed semivowels ([2504]). To these may be added also the liquids l and r. Contact of the semivowels i and u with their corresponding vowels i and u is avoided in classical times. See for -vu- [107, c]; for -quu- [157]; and for -i̭i- [104, c] (on obi̭iciō); [458] (Bōī for *Bōi̭ī). See [153, 3].
[PRONUNCIATION OF CONSONANTS.]
[53]. Most of the consonants are pronounced as in English. The following points must be noticed: