The PG diphthongs — ei ai iu eu au underwent regular independent changes in Early OE; they took place in all phonetic conditions irrespective of environment. The diphthongs with the i-glide were monophthongised into [i] and [a], respectively; the diphthongs in u were reflected as long diphthongs |io:|, leo:l and lea: I
All Engl dipthongs were monophonized from OE to NE. In PG there were no diphthongs. There was just a sequence of two separate vowels. Diphthongs appeared in OE: some (usually long diphthongs) – as a result of merging of two vowels: a + u à ea:, e + u à eo:, (i + u)à (io:) (dialectal variant). Others (usually short diphthongs) – as a result of the influence of the succeeding and preceding consonants (breaking of [æ, e]).
Breaking: Under the influence of succeeding and preceding consonants some Early OE monophthongs developed into diphthongs. If a front vowel stood before a velar consonant there developed a short glide between them, as the organs of speech prepared themselves for the transition from one sound to the other. The glide, together with the original monophthong formed a diphthong. The front vowels [i] and [e] and the newly developed [æ], changed into diphthongs with a back glide when they stood before [h], before long (doubled) [ll] or [l] pJus another consonant, and before [r] plus other consonants, e.g.: OE deorc, NE dark. The change is known as breaking or fructure. Breaking is dated in Early OE, for in OE texts we find the process already completed. Breaking produced a new set of vowels in OE — the short diphthongs [ea] and [eo[ they could enter the system as counterparts of'the long [ea:], [eo: ] which had developed from PG prototypes. OE diphthongs turned into monophthongs in ME.
New diphthongs appeared due to vocalisation of [j], [γ] and [w]. These consonants turned into vowels ([i], [u] and [u] respectively) and became the glides of the new diphthongs.
The diphthong oi was of French origin. Lengthening and diphthongization in NE (17c) due to the vocalization of r.
After short vowels
ME: o+r=o: (NE); ME: a+r=a: (NE); ME: I,e,u+r=e: (NE); ME: shwa+r=shwa (NE). After long vowels: i:+r=aie:; e:+r= ie:; a+r=ee:; o:+r=o:;
The Great Vowel Shirt: Early NE witnessed the greatest event in the history of English vowels — the Great Vowel Shift, — which involved the change of all ME long monophthongs, and probably some of the diphthongs. Great Vowel Shift – the change that happened in the 14th – 16th c. and affected all long monophthongs + diphthong [au]. As a result these vowels were: 1)diphthongized; 2) narrowed (became more closed); 3)both diphthongized and narrowed.
The spelling remained unchanged.
lt should be obvious from the chart and the table that the Great Vowel Shift did not add any new sounds to the vowel system; in fact, every vowel which developed under the Shift can be found in Late ME. And nevertheless the Great Vowel Shift was the most profound and comprehensive change in the history of English vowels: every long vowel, as well as some diphthongs, were "shifted", and the •pronunciation of all the words with these sounds was altered
