Early Scots was the emerging literary language of the Northern Middle English speaking parts of Scotland in the period before 1450. The northern forms of Middle English descended from Northumbrian Old English. During this period, speakers referred to the language as "English" (Inglis, Ynglis, and variants).
Early examples such as Barbour’s The Brus and Wyntoun’s Chronicle are better explained as part of Northern Middle English than as isolated forerunners of later Scots, a name first used to describe the language later in the Middle Scots period.
Northumbrian Old English had been established in south-eastern Scotland as far as the River Forth in the 7th century and largely remained there until the 13th century, which is why in the late 12th century Adam of Dryburgh described his locality as "in the land of the English in the Kingdom of the Scots" and why the early 13th century author of de Situ Albanie wrote that the Firth of Forth "divides the kingdoms of the Scots and of the English."
Political developments in the 12th century facilitated the spread of the English language. Institutions such as the burghs first established by David I, mostly in the south and east of Scotland, brought new communities into the areas in which they were established. Incoming burghers were mainly English (especially from Northumbria, and the Earldom of Huntingdon), Dutch and French. Although the military aristocracy employed French and Gaelic, these small urban communities appear to have been using English as something more than a lingua franca by the end of the 13th century, although this may not be surprising as the area south of the Forth in eastern lowland Scotland was already English speaking and had been since Anglo-Saxon times. Although the population of the largest burghs would have been counted in hundreds rather than thousands, a radical social shift occurred whereby many Gaels became assimilated into the new social system and its language.