seventieth (adj., n.)

"seven times ten; the number which is one more than sixty-nine; a symbol representing this number;" Old English (hund)seofontig, from seofon (see seven) + -tig (see -ty (1)). Similar formation in Old Saxon sibuntig, Old Frisian soventich, Middle Dutch seventich, Old High German sibunzug, Old Norse sjautugr.

Seventy-eight (78) "gramophone record played at a speed of seventy-eight revolutions per minute," which was the standard until the introduction of long-play (see LP) in 1948; the use of seventy-eight to distinguish the old discs is by 1951.

word-forming element making ordinal numbers (fourth, tenth, etc.), Old English -ða, from Proto-Germanic *-tha- (cognates: Gothic -da, -ta, Old High German -do, -to, Old Norse -di, -ti), from PIE *-to-, also *-eto-, *-oto-, suffix forming adjectives "marking the accomplishment of the notion of the base" [Watkins].

Cognate with Sanskrit thah, Greek -tos, Latin -tus; Sanskrit ta-, Lithuanian and Old Church Slavonic to, Greek to "the," Latin talis "such;" Greek tēlikos "so old, of such an age," Old Church Slavonic toli "so, to such a degree," toliku "so much," Russian toliko "only;" also see -ed.