Charles-Edwards (2013)

De Arbres
  • Charles-Edwards, T. M. 2013. Wales and the Britons 350–1064, Oxford university Press.

 "p. 92
 … the shift of the stress from the final to the penultimate syllable—occurred at much the same time in Welsh, Cornish, and Breton. Moreover, it did so as late as the ninth century.
 The date given by Jackson (1953:§§ 13, 207), is even later, in the eleventh century. Watkins (1972) & Watkins (1972-73) &  id. Watkins (1974-76), prefers an earlier date; Jackson (1975-76), maintains his earlier view; Koch (1997:cxxxvii–cxxxviii) has his own theory; Schrijver (1998–2000:147–55), argues for a date no later than the ninth century; Sims-Williams (2003:289) follows Schrijver."