 Difficulties arise otherwise in determining what should and what should not be included.Certain names are encountered frequently and particular care should be taken to differentiate between the various occurrences of Clydach, Clywedog and of Dulas for example as well as those whose names refer perhaps to the colour of their waters e.g.Afon "Goch" ("red" river), Afon "Ddu" ("black/dark" river) and Afon "Wen" ("white/light" river).