The names of rulers in present-day Wales found in the Matter of Britain are primarily derived from Geoffrey of Monmouth's . Written about 1136, this is a pseudohistorical account of British history, drawn largely from Geoffrey's vivid imagination, though also including some elements which were taken from older sources, such as the . It includes the names of many rulers, some of whom are based on real people, though the stories that Geoffrey told about them are wildly inaccurate. Geoffrey's fictions were accepted as historical truth, and became the centrepiece of the Matter of Britain, a cycle of frequently reworked medieval tales.
In the , Geoffrey initially defines Wales as one kingdom, Cambria, which he claims to be named after its first king, Brutus of Troy's son Camber. Only one more king of Cambria is named in the : Rudaucus, several generations later. Though the term Cambria, referring to the entirety of Wales, is occasionally used later in the book, it is divided into two kingdoms: Venedotia, defined by Geoffrey as North Wales, and Demetia, defined as South Wales. These are the historical Latin names of the kingdoms of Gwynedd and Dyfed respectively, though in reality they were only two of several Welsh kingdoms.
Variations of Geoffrey's were used to dignify the genealogies of many Welsh princes, nobles, and gentry. Known examples include Gruffudd ap Cynan. In the early 15th century, Owain Glyndà µr claimed the title Prince of Wales as a direct descendant and heir of King Camber, and also of the real medieval king Cadwaladr. In 1607, the Welsh genealogist John Williams recorded the claimed genealogies of some Welsh gentry families in his Book of Baglan. The more distant "ancestors" in these lineages vary between families, but they depend heavily on Geoffrey's fictions.
In his Welsh Classical Dictionary of 1993, Peter Bartrum collates names of these legendary (and real) rulers. He supplies dates for them, appropriate for their fictitious existence. These legendary dates are of no independent historical significance.