Overstrike

Typewriters, teletypes and impact printers had three tools: the hammer, the ribbon, and the option of not moving. The BS control character (ASCII 8) stepped back one position, and whatever came next landed on top of what was already there. Two strikes on the same letter gave bold, an underscore over a letter gave underlining, an apostrophe over an e gave é, and a slash over a zero gave the zero that cannot be mistaken for an O. Line printers did the same thing a whole line at a time with the + carriage control character.

Unix inherited the convention and never let go of it. nroff and man still emit c\bc for bold and _\bc for underline, which is precisely why col -b exists for when the output has to become a file. On hardcopy terminals the trick was also security: the login prompt printed a tangle of overstruck characters and invited you to type your password on top of them, paper having no way to blank itself.