Examples: cartography card archaeology archetype monarchy legal loyal purse personal cinema kinase car carpenter hippopotamus Mesopotamia dative data date
...
Proto-Celtic *to-

→ Welsh i (mention)
→ Welsh darllen (derived)
→ Irish tomhais (derived)
→ Irish táin (derived)
→ Irish turgnamh (derived)
→ Scottish Gaelic tomhais (derived)
→ Old Irish tomus (derived)