The Matter of Britain is a group of literature dealing with British Mythology. It includes the texts on famous kings of Britain, including the famous King Arthur. The Matter of Britain is sometimes paired with the Matter of France and the Matter of Rome. The literature is an example of classical mythology and medieval literature.
The Matters were first described in the French poet Jean Bodel's poem, Chanson de Saisnes, in the 12th century.
"There are but 3 matters that no man should be without, That of France, of Britain, and of great Rome."
The Matter includes famous kings of Britain, not just of Arthur, but of Brutus, Lear, and many others that were legendary.
The Matter includes the Knights of the Round Table, and features many other figures, for example the terrible Mordred and the wise Merlin.