How did North Germanic dialects differ between eastern and western Scandinavia?

During the Viking Age (c. 750–1050), North Germanic dialects began to diverge due to the differing orientations of Scandinavian kingdoms, leading to a distinction between a more conservative West Scandinavian area (Norway and its colonies, such as Iceland) and a more innovative East Scandinavian area (Denmark and Sweden).1

Key linguistic differences between these regions include:

• Monophthongization: A primary feature of the East Scandinavian dialect area was the monophthongization of the Old Scandinavian diphthongs ei, au, and øy into ē and ø. For example, the Old Scandinavian steinn ("stone") became stēn in the east, while the diphthongs were preserved in the west, as well as on the island of Gotland and in most North Swedish dialects.1
• Pronoun Development: The pronoun ek ("I") underwent a shift in East Scandinavian, becoming jak (modern Danish jeg, Swedish jag). In contrast, it remained ek in West Scandinavian (modern Icelandic ég, Faroese and New Norwegian eg).1

While these regional differences emerged, it is important to note that boundaries between dialect areas were often gradual rather than abrupt, and during the expansion period, all Scandinavians could generally communicate without difficulty.12