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-tr-, -tero-, -ter, -dor Relic of a comparative suffix.
A suffix in Proto-Indo-European and still used in Proto-Italic (cf. Spanish redor < Proto-Italic *wre-tero-s "towards the back"). In Latin, the suffix was no longer productive but still identifiable, occuring in words like dexter, minister, and magister (whence derecho, maestro, and even madre). From Proto-Italic *-tero- 'id.' From Proto-Indo-European *-tero- 'id.'
Indo-European
Italic
Oscan altrei "at another" (< *al-tero-), Umbrian putrespe "of both" (< *po-tero-kwid)
Celtic
Old Irish -thir, -ser, Old Welsh -ser, Middle Breton -zer, Old Cornish -ser
Germanic
Gothic -þar, Old Norse -tr, Old High German -der, Middle Dutch -ter, Old English -ðer (English -ther), Old Frisian -der
Balto-Slavic
Old Lithuanian -taras, Latvian -trs, Old Church Slavonic -terъ, Upper Sorbian -tery
Hellenic
Ancient Greek -τερός (terós)
Indo-Iranian
Sanskrit -tará, Avestan -āra-