In 1237 AD Scotland and England agreed a border on the River Tweed. Over three hundred years of history lay behind the decision.
In 900 AD the land of middle Britain from the Forth to the Humber was the Anglo-Danish land of Northumbria. England pushed north; Scotland pushed south and Northumbria was squeezed out. The border might have settled on the Forth or Tyne or Tees or Humber but, in 1237, in the Treaty of York Scotland and England agreed to put the Tweed betwixt.
In retrospect it can be seen that the birth of the border had taken palce mroe than two hundred years earlier when Malcolm, King of Scots, and Owain, King of Strathclyde defeated Uthred of Bamburgh at the Battle of Carham in 1018AD. Much water flowed in the Tweed between 1018 and 1237.