Examples: cartography card archaeology archetype monarchy legal loyal purse personal cinema kinase car carpenter hippopotamus Mesopotamia dative data date
...
Breton -ien
← Proto-Brythonic *-onos (inherited)
← Proto-Brythonic *-onā (mention)
← Welsh -ion (cognate)
← Cornish -yon (cognate)
← Old Breton -ion (inherited)
← Old Breton -on (mention)

→ Welsh -ion (cognate)
→ Cornish -yon (cognate)