The phonological system of the Old English language underwent many changes during the period of its existence. These included a number of vowel shifts, and the palatalisation of velar consonants in many positions.
For historical developments prior to the Old English period, see Proto-Germanic language.
Phonetic transcription
Various conventions are used below for describing Old English words, reconstructed parent forms of various sorts and reconstructed Proto-West-Germanic (PWG), Proto-Germanic (PG) and Proto-Indo-European (PIE) forms: Forms in italics denote either Old English words as they appear in spelling or reconstructed forms of various sorts. Where phonemic ambiguity occurs in Old English spelling, extra diacritics are used (, , , , , , , , ). Forms between /slashes/ or [brackets] indicate, respectively, broad (phonemic) or narrow (allophonic) pronunciation. Sounds are indicated using standard IPA notation.
The following table indicates the correspondence between spelling and pronunciation transcribed in the International Phonetic Alphabet. For details of the relevant sound systems, see Proto-Germanic phonology and Old English phonology.
1Proto-Germanic had two allophones each: stops and fricatives . The stops occurred: following a nasal; when geminated; word-initially, for and only; following , for only. By West Germanic times, was pronounced as a stop in all positions. The fricative allophones are sometimes indicated in reconstructed forms to make it easier to understand the development of Old English consonants. Old English retained the allophony , which in case of palatalisation (see below) became . Later, non-palatalized became word-initially. The allophony was broken when merged with , the voiced allophone of .
Phonological processes
A number of phonological processes affected Old English in the period before the earliest documentation. The processes affected especially vowels and are the reason that many Old English words look significantly different from related words in languages such as Old High German, which is much closer to the common West Germanic ancestor of both languages. The processes took place chronologically in roughly the order described below (with uncertainty in ordering as noted).
Absorption of nasals before fricatives
This is the source of such alternations as modern English five, mouth, us versus German , , . For detail see Ingvaeonic nasal spirant law.
First a-fronting
The Anglo-Frisian languages underwent a sound change in their development from Proto-West-Germanic by which , unless followed by or nasalized, was fronted to . This was similar to the later process affecting short , which is known as Anglo-Frisian brightening or First Fronting (see below). Nasalized and the sequences , were unaffected and were later raised to , , (see below). (This may be taken to imply that a nasal consonant , caused a preceding long vowel to nasalise.) In the non-West-Saxon dialects of English (including the Anglian dialect underlying Modern English) the fronted vowel was further raised to : W.S. , (< Proto-West-Germanic *slāpąn, *skāpă < Proto-Germanic *slēpaną, skēpą) versus Anglian slēpan, sċēp. The Modern English descendants sleep and sheep reflect the Anglian vowel; the West Saxon words would have developed to *sleap, *sheap.
The vowel affected by this change, which is reconstructed as being a low back vowel ā in Proto-West-Germanic, was the reflex of Proto-Germanic . It is possible that in Anglo-Frisian, Proto-Germanic /ɛː/ simply remained a front vowel, developing to Old English ǣ or ē without ever passing through an intermediate stage as the back vowel [ɑː]. However, borrowings such as Old English strǣt from Latin strāta (via) and the backing to ō before nasals are much easier to explain under the assumption of a common West Germanic stage *ā.
Monophthongization
Proto-Germanic /ai/ was monophthongized (smoothed) to (). This occurred after first a-fronting. For example, Proto-Germanic *stainaz became Old English stān (modern stone) (cf. Old Frisian stēn vs. Gothic stáin, Old High German stein). In many cases, the resulting was later fronted to by i-mutation: dǣlan "to divide" (cf. Old Frisian dēla vs. Gothic dáiljan, Old High German teilen [Modern English deal]). It is possible that this monophthongization occurred via the height harmonisation that produced the other diphthongs in Old English (presumably through an intermediate stage: > > ).