Etymology
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LP 

1948, abbreviation of long-playing phonograph record.

The most revolutionary development to hit the recording industry since the invention of the automatic changer is the Long Playing record, which can hold an entire 45-minute symphony or musical-comedy score on a single 12-inch disk. ... The disks, released a few weeks ago by Columbia Records and made of Vinylite, have phenomenally narrow grooves (.003 of an inch). They are played at less than half the speed of the standard old-style records. [Life magazine, July 26, 1948]
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seventy (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.

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