Pre-PDP11, DEC had machines with 12, 18, and 36 bit words. Those are all multiples of 3, so octal was more natural. (For the 16-bit PDP11, the instruction forma

Pre-PDP11, DEC had machines with 12, 18, and 36 bit words. Those are all multiples of 3, so octal was more natural. (For the 16-bit PDP11, the instruction format made octal natural.)