King’s College 2004 v Mistress Gunnvör sílfrahárr
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Overview: Languages in Viking Age Scandinavia
*Old West Norse vs. Old East Norse
vFrom the start of the Viking Age (ca. 800 A.D.) there began to be noticeable differences in pronunciation between Norway and the Norse colonies in the North Atlantic vs. Sweden, Denmark, and colonies in the Baltic.  These differences are enough that scholars recognize two dialects of Old Norse, Old West Norse (Norway, Iceland, Greenland, etc.) and Old East Norse (Sweden, Denmark).  There still wasn't a lot of difference, it was more like the difference between British English vs. American English.
*Continental Languages Cause Changes in Old East Norse
vOver time, Sweden and Denmark had a lot of direct trade and influence from the Continent, particularly from Germany, and these influences led to changes in pronunciation. Norway saw some changes in its language, but Icelandic (far away in the Atlantic) changed very little. By 1250 A.D. or so, the Scandinavian languages had diverged enough that one can term them from this point Old Icelandic, Old Norwegian, Old Swedish, and Old Danish.