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 .