William the Conqueror
In August 1086 William I summoned his tenants-in-chief and "landowning men of any account to William I, 'The Conqueror'" to Old Sarum where they swore allegiance to him and to be faithful against all other men. Thos event is known as The Oath of Salisbury. It was was demanded at a time of crisis when 'The Conqueror' was facing revolt and invasion. There seems little doubt that it was intended as a practical assurance and reminder rather than as a constitutional statement.