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 ?nig (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:]>[?:].
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 ?nig).
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, ?:], which arose through palatal mutation, merged with [?:] 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.)