The PG short [a] and the long [a:], which had arisen in West and North Germanic, underwent similar alterations in Early OE they were fronted and, in the process of fronting, they split into several sounds.
The principal regular direction of the change - [a]>[æ] and [a:]>[æ:] – is often referred to as the fronting or palatalisation of [a, a:]. The other directions can be interpreted as positional deviations or restrictions to this trend: short [a] could change to [o]or [a] and long [a:] became [o:] before a nasal; the preservation (or, perhaps, the restoration) of the short [a ] was caused by a back vowel in the next syllable— see the examples in Table 1 (sometimes [a] occurs in other positions as well, e.g. OE macian, land, NE make, land).
Splitting of [a] and [a:] in Early Old English
other OG languages
O Icel dagr
O Icel land
OI cel mánaðr
Development of Diphthongs
The PG diphthongs (or sequences of monophthongs) [ei, ai, iu, eu, au] — uderwent regular independent changes in Early OE; they took place in all phonetic conditions irrespective of environment. The diphthongs with i-glide were monophthongised into [i:] and [a:], respectively; the diphthongs in u-glide were reflected_a&_long__diphthongs [io:], [eo:] and [au] >[ea:].
If the sounds in PG were not diphthongs but sequences of two separate phonemes, the changes should be defined as phonologisation of vowel sequences. This will mean that these changes increased the number of vowel phonemes in the language. Moreover, they introduced new distinctive features into the vowel system by setting up vowels with diphthongal glides; henceforth, monophthongs were opposed to diphthongs.
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.