Old Irish cethair

← Proto-Celtic *kʷetwares (inherited)

← Proto-Indo-European *kʷetwóres (inherited)

→ Irish ceathair (inherited)

→ Irish ceathrú (mention)

→ Scottish Gaelic ceithir (inherited)

→ Scottish Gaelic ceathramh (mention)

→ Manx kiare (inherited)

→ Manx kerroo (mention)

→ Old Irish cethrar (affix)