^Peter Schrijver (1997),Studies in the History of Celtic Pronouns and Particles (Maynooth Studies in Celtic Linguistics; II), Maynooth: The Department of Old Irish, National University of Ireland,→ISBN,§ II.2.1, pages60–61
^Stüber, Karin (2017), “The morphology of Celtic”, in Jared Klein, Brian Joseph, Matthias Fritz, editors,Handbook of Comparative and Historical Indo-European Linguistics, volume 2, De Gruyter Mouton, page 1209 of 1203–1217
^R. J. Thomas, G. A. Bevan, P. J. Donovan, A. Hawke,et al., editors (1950–present), “hwy”, inGeiriadur Prifysgol Cymru Online (in Welsh), University of Wales Centre for Advanced Welsh & Celtic Studies
