> e According to Campbell, a-mutation of i is limited in Old English to just three words: nest "nest", spec "bacon", and wer "man". More plentiful instances of > have been cited in other West Germanic languages, with Old High German showing the greatest number of examples, including doublets such as skif : skef "ship". The mutation is rare in Old Norse, e.g. verr "man", heðan "hence", neðan "from below" in contrast to niðr "down(wards)" and perhaps jafn "even". Instances where a-mutation has failed to occur in Old Norse can mostly be explained as analogical forms, although a palatal stop or immediately preceding the in a short root syllable has a tendency to block or reverse the process. u > o While Proto-Germanic inherited both of the phonemes and from Proto-Indo-European, all instances of in the later languages arose from a-mutation of , since Proto-Indo-European had already become Proto-Germanic . a-mutation of is much more common than that of but also subject to many exceptions. In some dialects, the change may be blocked in labial contexts. Specifically, a tendency has been observed for the mutation not to occur next to initial or medial or in association with . Other exceptions, in particular where there is disagreement between dialects, may be due to the word having once been a u-stem. Most dialects of late Old Dutch underwent a merger of and , so that in Middle and Modern Dutch only appears, eliminating all traces of a-mutation of . The effects of a-mutation are perhaps most noticeable in certain verb types, e.g. strong verbs of classes 2, 3 and 4, where o in the past participle alternates with u in the preterite plural. For example, Old English flogen "flown" < *fluganaz alternated with flugon "they flew" < *flugun. Otherwise, where and would originally have alternated morphologically, the old Germanic languages had almost always generalised one vowel or the other throughout the paradigm, although there does occur in Old Swedish (especially in the laws of Östergötland) traces of regular alternation between and in line with a-mutation, e.g. kona (subj.) : kunu (obj.) "woman". As can be seen from the examples above, a-mutation is also found in lexical alternations. The diphthong In the West Germanic variety that gave rise to Old English, a-mutation did not affect the second element of the diphthong (for which the earliest Old English texts have eu): treulesnis "faithlessness", steup-