>Why is it 7-bit and not 8-bit?

To all the other history cited, there was also an early digital telephony multiplexing ("T1") practice of the day called "robbed bit signalling"[1] [2]. You got 7 bits clean, 8 bits - not so much.

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[1] https://en.wikipedia.org/wiki/Robbed-bit_signaling

[2] https://en.wikipedia.org/wiki/8-bit_clean