I-mutation is particularly visible in the inflectional and derivational morphology of Old English since it affected so many of the Old English vowels. Of 16 basic vowels and diphthongs in Old English, only the four vowels "ǣ, ē, i, ī" were unaffected by i-mutation. Although i-mutation was originally triggered by an or in the syllable following the affected vowel, by the time of the surviving Old English texts, the or had generally changed (usually to ) or been lost entirely, with the result that i-mutation generally appears as a morphological process that affects a certain (seemingly arbitrary) set of forms. These are most common forms affected: