"Duke of Cornwall" appears as a title in pseudo-historical authors such as Geoffrey of Monmouth. The list is patchy and not every succession was unbroken. Indeed, Monmouth repeatedly introduces Dukes of Cornwall only to promote them to the Kingship of the Britons and thus put an end to their line as (merely) dukes. As adjuncts or supporting roles to the kings of the Britons, the legendary dukes of Cornwall are considered part of the vast Matter of Britain, and can also be found in other stories, such as Culhwch and Olwen, the Prose Tristan, Havelok the Dane, and Gesta Herewardi. Antiquaries such as Richard Carew (Survey of Cornwall, 1602) and John Williams (the Book of Baglan, 1600–1607) also provide lists of legendary rulers of Cornwall, often combining the above with other sources.

As a result, these lists are more often thought of as a conglomeration of various Celtic rulers, Celtic warlords, and mythical heroes. If the lists of kings of Britain are legendary, then the list of dukes must be considered still more a genealogical and historical legend with no solid basis in the view of most historians. The titles given for the rulers also vary, even within sources; Monmouth's History, has the title fluctuating between "duke" (dux Cornubiae) and "king" (rex Cornubiae), and Carew wrote that before the Norman Conquest "these titles of honour carry a kinde of confusednes, and rather betokened a successive office, then an established dignity. The following ages received a more distinct forme, and left us a certeyner notice."^[1]