Have you ever wondered about the Unix terminal control keys: <kbd>Ctl</kbd>-<kbd>C</kbd> to interrupt a program, or Ctl-D to close the terminal? Do you know what Ctl-L does? Why does your computer beep if you press Ctl-G, but not other control keys?

The answer is that by holding down the Control key and typing a letter, you're sending a “control character” to the computer.

But what are these control characters, and why is there a seemingly random association between letters of the alphabet, and functions of the terminal?

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Here is the third part of my blog series expanding on my 4-bit rules of computing.

In this installment: Rules 5, on comments. Rule 5 is a bit contentious and I've taken too long in writing my thoughts on it — which is probably telling. Nonetheless I still want to press ahead and get these words out. I also wanted to include Rule 6 with this post, but I'm taking my own advice and breaking the post into two, because it really was getting quite long.

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