Most of the placenames are in Old Welsh (pre-12th C.) at the latest, though may in places may go back to late Brythonic (pre-7th C.). Of course, an early spelling doesn't necessarily mean the manuscript was written at the time. In the absence of any written place-record to ossify placenames, as our current maps do, it might be expected that contemporary spelling would be used. However, we know from the Book of Llan Dâv (See Other Works) that parish boundary documents recorded placenames from at least the Old Welsh period onwards.

Here are some of the pertinent spelling variations (from Strachan*, unless otherwise indicated):

Examples of dictation and transcription errors can be found in classic Welsh texts, for example, Culhwch ac Olwen (* page xvi), so we should be little surprised to find them here, though we await Vol.10 of Dumville's The Historia Brittonum (see Translations) which he notes* will contain a full study by K.H.Jackson of the Welsh language forms.