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 [ɔ] or [ā] and long [a:] became [ɔ:] 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).
Development of Diphthongs
The PG diphthongs (or sequences of monophthongs) − [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 ί-glide were monophthongised into [i:] and [a:], respectively; the diphthongs in -u were reflected as long diphthongs [io:], [eo:] and [ea:]. (The changes are shown in Table 2). 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 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:ə], with the second element weakened. This weakening as well as the monophthongisation of the sequences in [-i] may have 77 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 rād (NE road) and OE rǽd ('advice') which had not fallen together because while [ai] became [a:] in rād, the original [a:] was narrowed to [æ:] in the word rǽd. 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, θ or s]), and the subsequent growth of symmetrical oppositions in the sets of long and short vowels.