Growth of new diphthongs in Middle English
Thus in ME there appeared four new diphthongs: [ai], [ei], [au], [ou]. In addition to the diphthongs which developed from native sources, similar diphthongs. – with i- and u-glides – are found in some ME loan-words, e.g. i] in ME boy, joy, [au] in ME pause, cause ['pauzə, 'kauzə]. The diphthong [au] occurred also in French borrowings before a nasal, in imitation of Anglo-Norman pronunciation, e.g. ME straunge.
The newly formed ME diphthongs differed from the OE in structure: they had an open nucleus and a closer glide; they were arranged in a system consisting of two sets (with i-glides and with u-glides) but were not contrasted through quantity as long to short.
In OE short vowels were phonemically opposed to long ones. OE short vowels had developed from PG short vowels, while long ones went back to long vowels or bi-phonemic vowel sequences.
In later OE and in Early ME vowel length began to depend on phonetic conditions.
The earliest development was a change (the lengthening / shortening) of vowels before some consonant clusters; it occurred in Early ME or in Late OE.
1)Short vowels were lengthened before a sonorant and a plosive (ld, nd, mb), e.g. OE wild > ME wīld [wi:ld] ‘wild’, OE comb >ME cōmb ‘comb’, OE sund > ME sūnd ‘sound’.
2)Long vowels were shortened before all other groups of two or more consonants, e.g. OE cēpte > ME kepte ['keptə] ‘kept’; OE hūsbanda > ME husband; OE bewildrian > ME bewildren [be'wildrən] ‘bewilder’.
3)In the 12th or 13th c. short vowels became long in open syllables. This mostly affected the open short vowels [e], [a] and [o], but sometimes it is also found in the close vowels, [i] and [u]. In the process of lengthening close vowels became more open, e.g., OE open > ME open [']:pən] ‘open’, wike > weke ['we:kə] ‘week’, nama > name ['na:mə] ‘name’.
In spite of some restrictions (e.g. no lengthening occurred in polysyllabic words and before some suffixes, e.g. OE bodi¥ > ME body ['bodi] ‘body’, the alteration affected many words (see Table 7.5).
Quantitative Vowel Changes in Late Old English and Early Middle English
System of Vowels in Late Middle English
To sum up the results of Early ME vowel changes the system of vowels in Late ME is given in Table 7.6.
Middle English Vowels (the Age of Chaucer, Late 14th c.)
As seen from the table, the system of vowels in Late ME was no longer symmetrical. The OE balance of long and short vowels had been disrupted and was never restored again.
The most important change in the consonant system is the development of the fricative consonant  and the affricates [±] and [®] from OE palatal consonants or consonant combinations. For instance:
[k’] > [±] cild child
[sk’] >  scip ship
[g’] > [®] bryc¥ bridge
The phoneme denoted in OE by the letter c had two variants: [k] – hard and [k’] – palatal; the former remained unchanged, the latter gave us a new phoneme, the phoneme [±].
The phoneme denoted by the letters g or c¥and which existed in four variants: [g’], [g] – in spelling c¥and [j], [γ] – in spelling ¥ had the following development:
[g’] > [®] bridge
[j], [γ] were vocalized: dæ¥ > dai, ¥iet > yet
bo¥a > boue, dra¥an > drauen
[g] remained unchanged: ¥ōd > good
4. Conclusions: changes in ME phonetic system
1. Vowels in the unstressed position were levelled.
2. The sounds [æ] and [y] dissapeared from the system of the language.
3. There were no long diphthongs.
4. New diphthongs of the ei, aitype appeared with the glide more close than the nucleus (as contrasted to OE with the glide more open than the nucleus).
5. No parallelism existed between long and short monophthongs different only in their quantity.
6. The quantity of the vowel (i.e. the shortening / length) became dependent upon the phonetic environment (i.e. its position in the word: a, o, e– always long in an open syllable or before ld, mb, nd;all vowels are always short before two consonants, with the exception of ld, mb, nd). Only in one position – in a closed syllable before one consonant vowels of any quantity could be found (wīsbut pig). A long and the corresponding short vowel became mere allophones of the same phoneme:
7. New affricate phonemes [±],[®] and the fricative  appeared in the system of the language.