@b0rk I'm not sure whether there's any available information regarding specifically why CTC chose 8 bits, but 8-bit bytes were generally becoming commonplace in the late 1960s, in part for IBM compatibility, and in part due to the growing popularity of the ASCII and EBCDIC character codes, which were 7-bit and 8-bit codes, respectively, and 8-bit peripheral devices, including e.g. 8-level paper tape and 9-track magtape (8 data bits).