The perceptual system increases its capacity by organizing inputs into "chunks." Coined by (?) Miller 1956.
> A man just beginning to learn [Morse] code hears each dit and dah as a separate chunk. Soon he is able to organize these sounds into letters and then he can deal with the letters as chunks. Then the letters organize themselves as words, which are still larger chunks, and he begins to hear whole phrases. I do not mean that each step is a discrete process, or that plateaus must appear in his learning curve, for surely the levels of organization are achieved at different rates and overlap each other during the learning process. I am simply pointing to the obvious fact that the dits and dahs are organized by learning into patterns and that as these larger chunks emerge the amount of message that the operator can remember increases correspondingly.
When it comes to remembering digits, the average person can repeat back a sequence of about nine digits, whether those digits are 0s and 1s, or the numbers 0 through 9. By chunking heard binary digits into groups of three, you can remember far more than nine binary digits, somewhere around 27. (It’s not clear from Miller’s description whether the person is remembering the pattern 1-0-1 or consciously translating it into the octal digit 5, remembering that, and then translating back when it’s time to recite the numbers.)