From Proto-Italic *ənter, from Proto-Indo-European *h₁entér (“between”). Cognates include Sanskrit अन्तर् (antár, “between, within, into”), Oscan 𐌀𐌍𐌕𐌄𐌓 (anter, “between”), Old Irish eter (“between”), Albanian ndër (“between, among, amid, throughout”), Old High German untar (“between”) and German unter (“among”). PIE adverb *h₁entér gave rise to the adjective *h₁énteros (“inner, what is inside”), whence also interior (“interior”) and intrā (“inside, within”). The change from instrumental/ablative and accusative to accusative only is caused by *-teros used adverbially.