Northumbrian Old English had been established in south-eastern Scotland as far as the River Forth by the 7th century. It remained largely confined to this area until the 13th century, continuing in common use while Gaelic was the court language.
English then spread further into Scotland via the burgh.
After the 12th century early northern Middle English began to spread north and eastwards. It was from this dialect that Early Scots, known to its speakers as "English" (Inglis), began to develop, 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 thought that the Firth of Forth "divides the kingdoms of the Scots and of the English" .