Old English/I-mutation

What is i-mutation?


I-mutation is a regular change in the vowels stems of words under certain circumstances, surviving in modern English is such word-pairs as "full" and "fill", "blood" and "bleed", "mouse" and "mice".

In general, we may say that i-mutation of a vowel changes it to a vowel which is articulated further forward or higher up in the mouth than the original. The exact relationship between the original vowel and its mutant form is summarized in the table below.

original i-mutant
a, ā æ, ǣ
But a + m or n e + m or n
æ e
e i
o, ō e, ē
u, ū y, ȳ
ea, ēa ie, īe
eo, ēo ie, īe

Some things to note:

  • Vowels not on the list undergo no changes.
  • I-mutation never changes the length of a vowel or diphthong.
  • æ and e only change when they are short.
  • The behavior of a depends on whether or not it precedes a nasal consonant (m or n).

To make this easier to understand and to memorize, we can observe a few rules:

  • The back vowels (o/u/a and their long versions) become their fronted equivalents (e/y/æ).
  • The front vowels (a, e) all become more closed, i.e., they move closer to i/y, except ǣ and ē do not mutate.
  • All diphthongs (io (early)/eo/ea) mutate into ie/īe (short diphthongs to the former, long diphthongs to the latter).

Some uses of i-mutation


I-mutation can be seen in the following places, among others. It is not necessary for you to learn all of these instances now, as you can study them when they come up in articles on verbs, nouns, adjectives, etc; however, it will be useful for you to read through this section and familiarize yourself with the concepts in it.

  • Stem changes of certain nouns.

This survives in such Modern English nouns as "man"/"men", "tooth", "teeth", "mouse"/"mice", from the Old English "mann"/"menn", "tōþ"/"tēþ", "mūs"/"mȳs".

Old English also exhibited this feature in nouns which are now regular in English, such as "bōc"/"bēċ" (Modern English "book"/"books").

  • Nouns converted from adjectives by addition of þ.

A number of such nouns are still used in English: for example from "strong" we get "strength": compare Old English "strang"/"strengþ". Other examples where the effects of i-mutation are visible include the adjective "hāl" ("whole", "hale") forming the noun "hǣlþ" ("health"); and the word "fūl" ("foul") forming the noun "fȳlþ" ("filth").

  • 2nd and 3rd person singulars of strong verbs.

Compare, for example "iċ ċeorfe" ("I cut") with "þū ċierfst" ("thou cuttest"/"you (sg) cut") and "hē ċierfþ" ("he cuts"). Or again, from the verb "dūfan" ("to dive") we have "iċ dūfe", but "þū dȳfst" and "hē dȳfþ".

  • Formation of class 1 weak verbs from nouns and adjectives.

Why is it that you feed people food, and that you make things full by filling them? By now you should be able to recognize these as instances of i-mutation. In the first instance, the verb "fēdan" has been formed from the noun "fōda"; in the second instance, the verb "fyllan" has been formed from the adjective "full".

  • Some adjectives in the comparative and superlative.

For example, from the adjective "lang" ("long") we get the comparative "lengra" ("longer") and the superlative "lenġest" ("longest"). Note however that most adjectives do not work like this.

In Modern English this feature survives only in the existence of the words "elder", "eldest" as alternatives to "older", "oldest", from Old English "eald", "ieldra", "ieldest".