Magna Carta
The content of the Carta was designed to re-balance power between the King and his subjects, but especially between John and the barons. When King John set his seal on Magna Carta he conceded the fundamental principle that even as king he was not above the law.  That meant kings would not have as much power as they did before.  The Carta also laid down rules of inheritance, and that convictions required some kind of official process. It stated that people had rights not to be unlawfully imprisoned. In other words, the king is bound to rule within the law.