All the changes described above were interconnected. Their independence has been interpreted in different ways.

The changes may have started with the fronting of [a] (that is the change of [a] to [æ]), which caused a similar development in the long vowels: [a:]>[æ:], and could also bring about the fronting of [a] in the biphonemic vowel sequence [a + u], which became [æa:], or more precisely [æ: :], with the second element weakened. This weakening as well as the monophthongisation of the sequences in [-i] may have been favoured by the heavy stress on the first sound.

According to other explanations the appearance of the long [a:] from the sequence [a+i] may have stimulated the fronting of long [a:], for this latter change helped to preserve the distinction between two phonemes; cf. OE rod (NE road) and OE ræd ('advice') which had not fallen together because while [ai] became [a:] in rad, the original [a:] was narrowed to [æ:] in the word ræd. In this case the fronting of [a:] to [æ:] caused a similar development in the set of short vowels: [a] > [æ], which reinforced the symmetrical pattern of the vowel system.

Another theory connects the transformation of the Early OE vowel system with the rise of nasalised long vowels out of short vowels before nasals and fricative consonants ([a, i, u] plus [m] or [n] plus [x, f, 0 or s]), and the subsequent growth of symmetrical oppositions in the sets of long and short vowels .

Assimilative Vowel Changes: Breaking and Diphthongisation

The tendency to assimilative vowel change, characteristic of later PG and of the OG languages, accounts for many modifications of vowels in Early OE. 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], [e] and the newly developed [æ], changed into diphthongs with a back glide when they stood before [h], before long (doubled) [ll] or [l] plus another consonant, and before [r] plus other consonants, e.g.: [e]>[eo] in OE deorc, NE dark. The change is known as breaking or fracture. Breaking is dated in Early OE, for in OE texts we find the process already completed: yet it must have taken place later than the vowel changes described above as the new vowel [æ], which appeared some time during the 5th c., could be subjected to breaking under the conditions described.

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.

Breaking was unevenly spread among the OE dialects: it was more characteristic of West Saxon than of the Anglian dialects (Mercian and Northumbrian); consequently, in many words, which contain a short diphthong in West Saxon, Anglian dialects have a short monophthong, cf. WS tealde, Mercian talde (NE told).

Diphthongisation of vowels could also be caused by preceding consonants: a glide arose after * palatal consonants as a sort of transition to the succeeding vowel.

After the palatal consonants [k‘], [sk‘] and [j] short and long [e] and [æ] turned into diphthongs with a more front close vowel as their first element, e.g. Early OE *scæmu>OE sceamu (NE shame). In the resulting diphthong the initial [i] or [e] must have been unstressed but later the stress shifted to the first element, which turned into the nucleus of the diphthong, to conform with the structure of OE diphthongs (all of them were falling diphthongs). This process known as "diphthongisation after palatal consonants" occurred some time in the 6th c.

Breaking and diphthongisation are the main sources of short diphthongs in OE. They are of special interest to the historians of English, for OE short diphthongs have no parallels in other OG languages and constitute a specifically OE feature.

The status of short diphthongs in the OE vowel system has aroused much discussion and controversy. On the one hand, short diphthongs are always phonetically conditioned as the)' are found only in certain phonetic environments and appear as positional allophones of respective monophthongs (namely, of those vowels from which they have originated). On the other hand, however, they are similar in quality to the long diphthongs, and their phonemic status is supported by the symmetrical arrangement of the vowel system. Their very growth can be accounted for by the urge of the system to have all its empty positions filled. However, their phonemic status cannot be confirmed by the contrast of minimal pairs: [ea], [æ], [a] as well as [eo] and [e] occur only in complementary distribution, never in identical phonetic conditions to distinguish morphemes; they also occur as variants in different dialects. On these grounds it seems likely that short diphthongs, together with other vowels, make up sets of allophones representing certain phonemes: [a, æ, ea] and [e, eo]. Perhaps the rise of short diphthongs merely reveals a tendency to a symmetrical arrangement of diphthongs in the vowel system, which was never fully realised at the phonemic level.

Palatal Mutation

The OE tendency to positional vowel change is most apparent in the process termed "mutation". Mutation is the change of one vowel to another through the influence of a vowel in the succeeding syllable.

This kind of change occurred in PG when [e] was raised to [i] and [u] could alternate with [o] under the influence of succeeding sounds.

In Early OE, mutations affected numerous vowels and brought about profound changes in the system and use of vowels.

The most important series of vowel mutations, shared in varying degrees by all OE languages (except Gothic), is known as "i-Umlaut" or "palatal mutation". Palatal mutation is the fronting and raising of vowels through the influence of [i] or [j] (the non-syllabic [i]) in the immediately following syllable. The vowel was fronted and made narrower so as to approach the articulation of [i]. Cf. OE an (NE one) with a back vowel in the root and OE ænig (NE any) derived from the same root with the root vowel mutated to a narrower and more front sound under the influence of [i] in the suffix: [a:]>[æ:].

Since the sounds [i] and [j] were common in suffixes and endings, palatal mutation was of very frequent occurrence. Practically all Early OE monophthongs, as well as diphthongs except the closest front vowels [e] and [i] were palatalised in these phonetic conditions.

Due to the reduction of final syllables the conditions, which caused palatal mutation, that is [i] or [j], had disappeared in most words by the age of writing; these sounds were weakened to [e] or were altogether lost (this is seen in all the examples above except ænig).

Of all the vowel changes described, palatal mutation was certainly the most comprehensive process, as it could affect most OE vowels, both long and short, diphthongs and monophthongs. It led to the appearance of new vowels and to numerous instances of merging and splitting of phonemes.

The labialised front vowels [y] and [y:] arose through palatal mutation from [u] and [u:], respectively, and turned into new phonemes, when the conditions that caused them had disappeared. Cf. mus and mys (from the earlier *mysi, where [y:] was an allophone of [u:] before [i]). The diphthongs [ie, ie:] (which could also appear from diphthongisation after palatal consonants) were largely due to palatal mutation and became phonemic in the same way, though soon they were confused with [y, y:]. Other mutated vowels fell together with the existing phonemes, e.g. [oe] from [o] merged with [e, æ:], which arose through palatal mutation, merged with [æ:] from splitting.

Palatal mutation led to the growth of new vowel interchanges and to the increased variability of the root-morphemes: "owing to palatal mutation many related words and grammatical forms acquired new root-vowel interchanges. Cf., e.g. two related words: OE gemot n 'meeting' and OE metan (NE meet), a verb derived from the noun-stem with the help of the suffix -j- (its earlier form was *motjan; -j- was then lost but the root acquired two variants: mot'/met-). Likewise we find variants of morphemes with an interchange of root-vowels in the grammatical forms mus, mys (NE mouse, mice), boc, bec (NE book, books), since the plural was originally built by adding -iz. (Traces of palatal mutation are preserved in many modern words and forms, e.g. mouse — mice, foot—feet, tale — tell, blood— bleed; despite later phonetic changes, the original cause of the inner change is t-umlaut or palatal mutation.)

The dating, mechanism and causes of palatal mutation have been a matter of research and discussion over the last hundred years.

Palatal mutation in OE had already been completed by the time of the earliest written records; it must have taken place during the 7th c., though later than all the Early OE changes described above. This relative dating is confirmed by the fact that vowels resulting from other changes could be subjected to palatal mutation, e. g. OE ieldra (NE elder) had developed from *ealdira by palatal mutation which occurred when the diphthong [ea] had already been formed from [æ] by breaking (in its turn [æ] was the result of the fronting of Germanic [a]). The successive stages of the change can be shown as follows: fronting - breaking - palatal mutation [a] > [æ] > [ea] > [ie] The generally accepted phonetic explanation of palatal mutation is that the sounds [i] or [j] palatalised the preceding consonant, and that this consonant, in its turn, fronted and raised the root-vowel. This "mechanistic" theory is based on the assumed workings of the speech organs_.. An alternative explanation, sometimes called "psychological" or "mentalistic", is that the speaker unconsciously anticipates the [i] and [j] in pronouncing the root-syllable – and through anticipation adds an. i-glide to the root-vowel. The process is thus subdivided into several stages, e.g. *domjan >*doimjan >*doemjan >*deman (NE deem). It has been found that some OE spellings appear to support both these theories, e.g. OE secgan has a palatalised consonant [gg‘] shown by the digraph cg; Coinwulf, a name in BEOWULF, occurring beside another spelling Cenwulf, shows the stage [oi:] in the transition from PG [o:] to OE [oe:], and [e:]: OE cen 'bold'. The diphthongoids resulting from palatal mutation developed in conformity with the general tendency of the vowel system: in Early OE diphthongal glides were used as relevant phonemic distinctive features. In later OE the diphthongs showed the first signs of contraction (or monophthongisation) as other distinctive features began to predominate: labialisation and vowel length. (The merging of [ie, ie:] and [y, y:] mentioned above, can also be regarded as an instance of monophthongisation of diphthongs.)

Changes of Unstressed Vowels in Early Old English

All the changes described above affected accented vowels. The development of vowels in unstressed syllables, final syllables in particular, was basically different. Whereas in stressed position the number of vowels had grown (as compared with the PG system), due to the appearance of new qualitative differences, the number of vowels distinguished in unstressed position had been reduced. In unaccented syllables, especially final, long vowels were shortened, and thus the opposition of vowels – long to short – was neutralised. Cf. OE nama (NE name) to the earlier *namon. It must also be mentioned that some short vowels in final unaccented syllables were dropped. After long syllables, that is syllables containing a long vowel, or a short vowel followed by more than one consonant, the vowels [i] and [u] were lost. Cf. the following pairs, which illustrate the retention of [u] and [i] after a short syllable, and their loss after a long one: OE scipu and sceap (NE ships, sheep, pl from *skeapu); OE werian—demon (NE wear, deem; cf. Gt domjan).

Old English Vowel System (9th-10th c.)

The vowels shown in parentheses were unstable and soon fused with resembling sounds: [a] with [a] or [o], [ie, ie:] with [y, y:].

The vowels are arranged in two lines in accordance with the chief phonemic opposition: they were contrasted through quantity as long to short and were further distinguished within these sets through qualitative differences as monophthongs and diphthongs, open and close, front and back, labialised and non-labialised. Cf. some minimal pairs showing the phonemic opposition of short and long vowels: