Commodore PET
Apple II
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Correct Answer: IBM PC 

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Although desktop computers existed prior to 1977, they tended to be too expensive for home users, that year marked a revolution personal computers with the arrival of three landmark machines: the Apple II, Commodore PET and Tandy TRS-80, which were offered pre-assembled with a monitor and keyboard. The IBM PC, however, wasn't released for another four years in 1981.

The "1977 Trinity" phrase was purportedly coined and popularized by American microcomputer magazine "Byte" (1975-1998), although after searching digital archives we haven't found the exact copy that contains the reference.

The Atari 400/800 and Texas Instruments TI99/4 joined the fray in 1979, by which point over half a million microcomputers had been sold.

Continue reading... The History of the Personal Computer