The British system of government is based on an uncodified constitution, meaning that it is not set out in any single document. The British constitution consists of many documents and most importantly for the evolution of the office of the prime minister, it is based on customs known as constitutional conventions that became accepted practice. In 1928, Prime Minister H. H. Asquith described this characteristic of the British constitution in his memoirs:In this country we live ... under an unwritten Constitution. It is true that we have on the Statute-book great instruments like Magna Carta, the Petition of Right, and the Bill of Rights which define and secure many of our rights and privileges; but the great bulk of our constitutional liberties and ... our constitutional practices do not derive their validity and sanction from any Bill which has received the formal assent of the King, Lords and Commons. They rest on usage, custom, convention, often of slow growth in their early stages, not always uniform, but which in the course of time received universal observance and respect.