In the terminology of linguistics, mutation is a change in a vowel sound caused by a sound in the following syllable. I-mutation is a change in the sound of a vowel so that it is pronounced with the tongue higher and farther forward. Because i-mutation happened very early in the development of Old English, and it affects all areas of the language, it is worth examining the linguistic reasons behind why it occurs, and the rules that govern it.

What causes i-mutation?

I-mutation was a series of changes to initial vowels which took place when there was a high front vowel, usually , or ) in the following syllable (in practice, almost always an inflectional suffix). The presence of /i/ caused lower and back vowels in the root of the word to change - to move higher - in anticipation of pronouncing this high front vowel at the end of the word. By the time Old English was spoken, the high vowel in the second syllable or inflectional suffix had either disappeared or changed to 'e', but by examining cognate words in related languages, like the Old English 'dælan - to divide' against the Gothic 'dailjan - to divide', we can see that the high vowel /j/ was there in an earlier form of the language. Essentially, the /i/ or /j/ of the suffix is no longer present in Old English, but its effects on the vowel in the root of the word are still there.

How does i-mutation work?

Vowels can be sorted into two categories: back vowels and front vowels. Back vowels are vowels produced with the tongue further back in the mouth like , , or , while front vowels are vowels produced with the tongue in the front of the mouth like , , or . It can be hard to visualise where a vowel sound is produced, but examine the below diagram and try to make the sounds and . You should feel your tongue move as you do this, with the origin of the sound shifting backwards and lower as you move from [i] to [o]. You can click on any of the dots in the vowel origin chart below to hear the related vowel sound.

Old English vowel origin chart a ɒ Y y ä Front vowel soundsBack vowel sounds

When a front vowel follows a stressed syllable, low front vowels are ‘raised’ higher in the mouth, and back vowels are ‘fronted’, produced further forward in the mouth. The reason for this is the mouth of the speaker gets ready for the high front sound too soon and in the process the tongue moves too early, changing the sound created. You can find a full breakdown of the vowel changes below. The long vowels , like in 'ærest' and , like in 'cwen' are never subject to i-mutation.

a æ
am em
æ e
e i
o e
u y
ea ie
eo ie

You have seen i-mutation at work already in the adjectives module when certain adjectives are made comparative and superlative. For example, 'heah' becomes 'hiera' and 'hiehst'. It is also visible in the singular forms of the irregular verbs 'gan' and 'don', where the vowel in the stem changes from 'a' to 'æ' in second and third person singular 'gæst'' and 'gæþ' and 'o' becomes 'e' in dest and deþ. The endings with /i/ have disappeared in second and third person, but the i-mutation of the vowel in the root of the word remains.

Nouns affected by i-mutation

I-mutation is visible in the root-vowel of the singular dative, nominative plural, and accusative plural forms of certain nouns, and many of the nouns affected by i-mutation in Old English still decline differently in the plural in modern English. For example, in the nominative plural 'fot - foot' becomes 'fet - feet', 'gos - goose' becomes 'ges - geese', and 'toþ - tooth' becomes 'teþ - teeth'. However, there are some notable exceptions which have i-mutation in Old English, but that have been lost in modern English. For example, Old English 'boc - book' becomes 'bec - books' and 'freond - friend' becomes 'friend - friends'.

Singular Plural
Nominative se mann þa menn
Accusative þone mann þa menn
Genitive þæs mannes þara manna
Dative þæm menn þæm mannum
Singular Plural
Nominative seo boc þa bec
Accusative þa boc þa bec
Genitive þære boce þara boca
Dative þære bec þæm bocum
Singular Plural
Nominative se freond þa friend
Accusative þone freond þa friend
Genitive þæs freondes þara freonda
Dative þæm friend þæm freondum

You can practice nouns affected by i-mutation below.

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