Originally the duchy was called Sønderjylland but in the late 14th century the name of the city Slesvig (now Schleswig) started to be used for the whole territory.

However, around 1830 some Danes started to re-introduce the archaic term Sønderjylland to emphasize the area's history before its association with Holstein and its connection with the rest of Jutland.

A central element of the German nationalistic claim was the insistence on Schleswig and Holstein being a single, indivisible entity (as they had been declared to be in the Treaty of Ribe 1460).

From early medieval times, the area's significance was its role as a buffer zone between Denmark and the powerful Holy Roman Empire to the south, as well as being a transit area for the transfer of goods between the North Sea and the Baltic Sea, connecting the trade route through Russia with the trade routes along the Rhine and the Atlantic coast (see also Kiel Canal).

Roman sources place the homeland of the tribe of Jutes north of the river Eider and that of the Angles south of it.

By the early Middle Ages, the region was inhabited by three groups: During the 14th century, the population on Schwansen began to speak Low German alongside Danish,[2] but otherwise the ethno-linguistic borders remained remarkably stable until around 1800, with the exception of the population in the towns that became increasingly German from the 14th century onwards.

[5] In 1115, King Niels created his nephew Canute Lavard – a son of his predecessor Eric I – Earl of Schleswig, a title used for only a short time before the recipient began to style himself duke.

[6] In the 1230s, Southern Jutland (the Duchy of Slesvig) was allotted as an appanage to Abel Valdemarsen, Canute's great-grandson, a younger son of Valdemar II of Denmark.

Feuds and marital alliances brought the Abel dynasty into a close connection with the German Duchy of Holstein by the 15th century.

These dual loyalties were to become a main root of the dispute between the German states and Denmark in the 19th century, when the ideas of romantic nationalism and the nation-state gained popular support.

Following the Protestant Reformation, when Latin was replaced as the medium of church service by the vernacular languages, the diocese of Schleswig was divided and an autonomous archdeaconry of Haderslev created.

On the west coast, the Danish diocese of Ribe ended about 5 km (3 mi) north of the present border.

Feudal lordship was combined with technical modernization, and the distinction between unfree labour and paid work was often vague.

The feudal system was gradually abolished in the late 18th century, starting with the crown lands in 1765 and later the estates of the nobility.

This linguistic change created a new de facto dividing line between German and Danish speakers north of Tønder and south of Flensburg.

This caused a conflict between Denmark and the German states over Schleswig and Holstein, which led to the Schleswig-Holstein question of the 19th century.

This was met by German states in two ways: The defeated Danish king had to leave Schleswig and Holstein to Austria and Prussia.

Following the Second World War, a substantial part of the German population in Southern Schleswig changed their nationality and declared themselves as Danish.

This change was caused by a number of factors, most importantly the German defeat and an influx of a large number of refugees from the former Prussian eastern provinces, whose culture and appearance differed from the local Germans, who were mostly descendants of Danish families who had changed their nationality in the 19th century.

However, the majority in the Danish parliament refused to support a referendum in South Schleswig, fearing that the "new Danes" were not genuine in their change of nationality.