Blake and Lloyd list the three keys to their understanding of the geography of sub-Roman Britain in Chapter 3 of their book. They cite Giraldus Cambrensis as an authority for the tripartite division of Britannia; they cite Enwau Ynys Brydain (which they misspell – a bad omen) for the names of the three chief rivers of Ynys Pridein; and the Welsh Brut as the source of the names of the three seas of Ynys Pridein. It is on these three ‘keys’ that their ‘new geography’ of Arthurian Britain rests.