-au

From Middle Welsh -eu, from Old Welsh -ou, from Proto-Brythonic *-ow, from Proto-Celtic *-owes, u-stem nominative plural.[1] Cognate with Cornish -ow and Breton -où.

• -iau

• (North Wales) IPA^(key): (standard) /aɨ̯/, (colloquial) /ɛ/, /a/
• (South Wales) IPA^(key): (standard) /ai̯/, (colloquial) /ɛ/

-au

1. Forms plural nouns., -s

   Synonyms: -aid, -aint, -ed, -edd, -en, -i, -iadau, -iaid, -iau, -ion, -od, -oedd, -on, -ydd, -yr, -ys

   ‎syniad (“idea”) + ‎-au → ‎syniadau (“ideas”)

   ‎methiant (“failure”) + ‎-au → ‎methiannau (“failures”)

   ‎gwasanaeth (“service”) + ‎-au → ‎gwasanaethau (“services”)

• -áu (when there is confusion with an identically-spelled plural)

• (North Wales) IPA^(key): /ˈaɨ̯/
• (South Wales) IPA^(key): /ˈai̯/

-au

1. Alternative form of -hau (forms verbal nouns)

• King, Gareth (2007). Modern Welsh Dictionary. Oxford University Press. →ISBN.
• R. J. Thomas, G. A. Bevan, P. J. Donovan, A. Hawke et al., editors (1950–present), “-au”, in Geiriadur Prifysgol Cymru Online (in Welsh), University of Wales Centre for Advanced Welsh & Celtic Studies