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The i-mutation was a vowel-mutation that has been observed to affect many Old English words.

Cause and Effect[]

First, to understand it better, you must be aware of the different parts of the mouth in which Old English vowels were pronounced. Here is a table which shows where the vowels were pronounced (vertical: low, central, high; horizontal: front, back):

Back Central Front
Top U I/y
Mid O E
Low A Æ

The i-mutation happened sometimes when the high front vowel (e.g. i) or its corresponding consonant (e.g. the y in yes) was after the main vowel of a word (often in suffixes); the speaker's mind would sometimes get ready to pronounce the high front vowel too soon and would affect how speaker said the main vowel, causing back vowels to shift forward to their corresponding front vowels (note that u would shift to y and keep its rounding, not i) and front vowels (if they could) would shift upwards (obviously i could not shift any further upwards, so it was unaffected), and often the i/y sound was lost. Here's a more detailed chart of the effect:

Original vowel Mutated vowel

Note that all of the back vowels, whether long or short, could be affected by the i-mutation, but only the short front vowels could be affected.

Note also that a, when before a nasal consonant (m or n) would "skip a corner" and shift to e, therefore:

Original vowel Mutated vowel
A(+m/n) E

The two diphthongs eo and ea shifted to ie:

Original vowel Mutated vowel
Ea Ie
Eo Ie