- -th
- suffix forming nouns (e.g. depth, strength, truth, etc.), from Old English -ðu, -ð, from PIE *-ita (cf. Sanskrit -tati-, Greek -tet-, Latin -tati-).
The suffix forming ordinal numbers (fourth, tenth, etc.) is Old English -ða, from PIE *-tos (cf. Sanskrit thah, Greek -tos, Latin -tus).