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):

• Modern and Middle Welsh Ll was written L at the start of Old Welsh words, eg. Lin Liuan but ll elsewhere. There was also a pronounciation change for this form from "L" to the current "Lth" in the 10th C.*.
• The "u" sound, written w in Modern Welsh was written u in Old Welsh, but also (along with v, 6 and w) in Middle Welsh. However, the "i" sound was already in Old Welsh starting to be written u. In Middle Welsh this was also written as v.
• The "i" sound, written y in Modern Welsh was written i in Old Welsh, but also (along with y, dotted-y, and e) in Middle Welsh. The "y" sound was also written i in Old Welsh, while in Middle Welsh it is i or y.
• The "w" sound in Old Welsh is written u, but in Middle Welsh w, 6, u, or v. In both wy is sometimes replaced with oy. The Old Welsh guo- becomes go-, while gu becomes "w", written 6, w, u, uu or v. The Brythonic sv becomes hw in Old Welsh, and chw in Middle Welsh; likewise w becomes the Old Welsh hw and Middle Welsh gw.
• p and b within words are pretty interchangable in Old and Middle Welsh.
• The Old Welsh c is later replaced with k in Middle Welsh, though c is also found.
• The "spirant-v" in Middle Welsh is written u, uu, v, fu, w, and, as in Modern Welsh f (pronouced "vh"). In Old Welsh this appears to be written m.

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.