Examples:
cartography
card
archaeology
archetype
monarchy
legal
loyal
purse
personal
cinema
kinase
car
carpenter
hippopotamus
Mesopotamia
dative
data
date
...
Breton
-où
←
Proto-Brythonic
*-ow
(inherited)
←
Proto-Celtic
*-owes
(inherited)
←
Welsh
-au
(cognate)
←
Cornish
-ow
(cognate)
→
Welsh
-au
(cognate)
→
Cornish
-ow
(cognate)