The Oath of Salisbury refers to an…

The Oath of Salisbury refers to an event in August 1086 when William I of England summons his tenants-in-chief and "landowning men of any account" to Salisbury, where they swear allegiance to him and to be faithful against all other men.

The oath is demanded at a time of crisis when the Conqueror is facing revolt and invasion.

There seems little doubt that it is intended as a practical assurance and reminder rather than as a constitutional statement.