-th

-th suffix forming nouns from verbs (depth, strength, truth, etc.), from O.E. -ðu, -ð, from PIE *-ita (Cf. Skt. -tati-, Gk. -tet-, L. -tati-). The suffix forming ordinal numbers (fourth, tenth, etc.) is O.E. -ða, from PIE *-tos (Cf. Skt. thah, Gk. -tos, L. -tus).