Scandinavian u-umlaut (u-mutation) manifests itself in very different forms. When weakly stressed u had been syncopated and transferred its phonetic value to a preceding stressed a, the language had acquired a new phoneme. This was preserved in some form in all the Scandinavian languages, but to varying degrees. In Iceland, the phoneme still occurs entirely regularly throughout the lexicon. In the rest of West Scandinavia, it was preserved in early medieval writing, but was later quite often replaced by a. In East Scandinavia, too, a is its commonest counterpart, but other vowels are also found. This is particularly true of Denmark. Here the umlaut phoneme has for the most part become a by vowel merger. In written Norwegian, a-forms reigned almost supreme by the late Middle Ages, but in the dialects of that language mutated forms survive to this day. Their distribution is uneven, but clearly shows that a-forms in Norwegian are later loans, with a dialect geographical link with corresponding words in East Scandinavian, especially Swedish. Individual words can occur with umlaut, without umlaut or in both forms, with considerable variation from one dialect to another. The outcome is related to the social status of the different forms: only umlautless forms that are fully accepted socially in East Scandinavia find their way into Norwegian dialects. By the end of the Middle Ages, written Norwegian had a-forms almost throughout, presumably reflecting a prestige language very strongly influenced by East Scandinavian. This development can be seen as an incipient koinéisation of the larger Scandinavian languages.
Uppsala: Institutionen för nordiska språk , 2010.