Examples:
cartography
card
archaeology
archetype
monarchy
legal
loyal
purse
personal
cinema
kinase
car
carpenter
hippopotamus
Mesopotamia
dative
data
date
...
Cornish
-ieth
←
Proto-Brythonic
*-aɨθ
(inherited)
←
Proto-Celtic
*axtā
(inherited)
←
Welsh
-aeth
(cognate)
←
Irish
-acht
(cognate)
→
Welsh
-iaeth
(cognate)