Old Norse is the language of the Vikings, sagas, runes, eddic and skaldic verse. The Norse language is still spoken by Icelanders today in a modern style.
Medieval Scandinavians called their Old Norse language the ‘Danish tongue,’ dǫnsk tunga. No one is quite sure why this was so. Perhaps it was because Denmark was the first of the Scandinavian lands to become a powerful, centralized kingdom, and the speech of the influential Danish court became for a time the accepted standard. It may also have been because the Danes were closest to the Frankish Empire and the rest of Europe.
Several questions concerning Old Norse arise. One is: How close was Old Norse to Old English? The answer is that Old Norse was related to, but different from, the language spoken in Anglo-Saxon England. With a little practice, however, Old Norse and Old English speakers could understand each other, a factor that significantly broadened the cultural contacts of Viking Age Scandinavians. The two languages derived from a similar Germanic source, which had diverged long before the start of the Viking Age.
Figure 1. Indo-European Languages arriving at Proto Old Norse, the stage of language that precedes Old Norse (illustrated in the next figure). (Jesse Byock, Viking Language 1: Learn Old Norse, Runes and Icelandic Sagas)
Another question is: Does learning Old Norse/Old Icelandic help in learning Modern Icelandic? The answer is that the two languages are quite similar. The Old Norse of the medieval Icelanders, especially what is found in the sagas, remains the basis of Modern Icelandic. Most of the grammar and vocabulary taught in this book are still current in Modern Icelandic.
As a distinct language, Old Norse has a traceable history. It is the most northerly and most westerly medieval member of the large Indo-European family of languages. The Indo-European language family tree offers an overview of the placement of Proto Old Norse (the ancestor of Old Norse) in the Germanic branch of Indo-European. Old Norse shares a close relationship with early Germanic languages such as Old English, Gothic, and Old High German, while the relationship with other Indo-European languages, such as Latin, Greek, and Sanskrit, is more distant.
Figure 2. Proto Old Norse (North Germanic) and Its Main Descendant Languages. Proto Old Norse (PN) developed into Old Norse (ON) by the 8th century. Several smaller languages can be added to the larger West Old Norse and East Old Norse languages in this chart. These include Faroese (the language of the Faroe Islands) under ‘West Old Norse’ and Old Gutnish (the language of the Baltic Island of Gotland) under ‘East Old Norse.’ Old Gutnish is in some ways its own language, but it also shares many similarities with East Old Norse. (Jesse Byock, Viking Language 1)
At the start of the Viking Age, there were two closely related varieties of Old Norse. East Old Norse was spoken in Denmark, Sweden, and the Norse Baltic region. West Old Norse was spoken in Norway and the Atlantic Islands, including the Hebrides, Orkney, Shetland, the Faroes, Iceland, and Greenland. Toward the end of the Viking period, around the year 1000, with accumulation of small changes, Old West Norse began to split into Old Icelandic and Old Norwegian.
Icelandic and Norwegian share an especially close kinship, since Iceland was settled largely by Old Norse speakers from mainland Norway and from the Norwegian Viking Age colonies in the British Isles. Today, we call the language of the sagas and the other written Icelandic sources Old Norse (ON) or more precisely Old Icelandic (OI). Old Icelandic is a branch of the Old West Norse that developed in Iceland and Norway from the Old Norse speech of the first Viking Age settlers.
By the 12th century, differences between Old Icelandic and Old Norwegian were noticeable but still minor, resembling to some extent present-day distinctions between American and British English. At roughly the same time, East Old Norse diverged into Old Swedish and Old Danish. Still, the four languages remained similar and mutually intelligible until about AD 1500, and all the Old Norse sources, from either the Atlantic or the Baltic regions, are accessible with training in Old Norse.
By the modern period, Norwegian, Swedish, and Danish changed considerably from Old Norse. These languages were strongly influenced by Low German dialects and English. They dropped numerous aspects of Old Norse grammar and changed many sounds. Modern Icelandic, however, remained faithful to the older language and Icelandic underwent remarkably few alterations.
Today, speakers of modern mainland Scandinavian languages can understand one other, but they cannot understand Icelandic. Old Icelandic grammar is very similar to Modern Icelandic grammar. The most noticeable diversions from the medieval language to the modern are a series of sound shifts, spelling modifications, and the adoption of new words and meanings.
— Jesse Byock