Nowadays, a default prompt in a Unix shell might look like % _ or $ _ (with the underscore indicating the cursor location), or, on other systems, a more elaborate string, like C:\>_.

On the , however, most widely used interactive environments used backspaced prompts on serial video terminals, typically (*), for the command mode, and with various other characters instead of the minus (e.g. a colon, an asterisk, etc) to indicate other input modes.

On hardcopy (Baudot) TTYs, the backspace character in such prompts would be replaced with a space.

Were the backspaced prompts just a local fad, or were there Western systems in the early 1970s known for that prompt style?

(*) Rendered as the hyphen followed by a backspace character. The backspace control character was non-destructive on the video terminals connected to the BESM-6. The notion that a backspace character always erases is mistaken; it is an artefact of modern OSes.

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    So the prompt included a character which was then removed by the following control character? Why? May 6, 2022 at 8:00
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    I have a hard time imagine what a 'Backspace-Prompt' may be. Could you maybe add a description what is visible and maybe what codes are send toward a terminal and or TTY?
    – Raffzahn
    May 6, 2022 at 8:03
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    @OmarL The backspace character was non-destructive.
    – Leo B.
    May 6, 2022 at 8:28
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    Is this another DC3 workaround where it was imitating a teletype which wanted to start typing in the middle of the screen so a dummy character (DC3) was output. These workaround stayed long after the paper terminals had been replaced by VDUs.
    – cup
    May 6, 2022 at 8:35
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    @LeoB. I know only underscore, as it was non destructive (the first character typed stayed visible), a clear indicator about state and as well quite useful when reading the printout later on. TTY is TTY, no matter what code set. On an ITA2 (even a Russian one), Underscore + BS would be replaced by Fig. + Minus + CR + Let., giving the same handling (albeit being more destructive).
    – Raffzahn
    May 6, 2022 at 9:20

2 Answers 2


(Caveat: This is all based on the faint memory a grey beard collected even before he had a beard at all, so no links this time)

It was a well known practice used to indicate request for input on a printing terminal without taking away line length. Keep in mind that handling of line breaks in input is even today a challenging task :)) Similar automatic line break/wrap to next line could not be expected on early terminals.

It essentially led to a printout looking more 'naturally' (*1) to the observer with all lines starting at the column 1, but input lines marked by an underlined first character. This preserved known text aesthetics, while allowing easy scann for 'who' wrote what. It also turned (later) into a kind of a convention to mark input lines as such by underlining them when printed out.

While I can't point out any specific OS, but have it seen on (real old) mainframe (/360) output as well.

*1 - Keep in mind, this is about a time before people were used to any UI idioms we take for granted nowadays. Before there was Letters, Books and at most TTY communication with no prompt inbetween.

  • I wonder if a system using the underline convention (or the "Fig. + Minus + CR + Let." convention) survives in an emulator to confirm that, or if there are illustrations of such printouts in books.
    – Leo B.
    May 6, 2022 at 16:12
  • @LeoB. Sorry, this time I have no lead for more information. Just my memory from way back in the 1970 (interestingly a remote console software of ca 1983/84 for BS2000 used as well the convention to underline everything entered by any operators, online as well as when scrolling thru a log. I was quite annoyed by that at first, but I can't tell if this is simply because underline was the least offensive markup for that terminal type, or someone in OS development still remembered/continued the old ways).
    – Raffzahn
    May 6, 2022 at 17:00
  • In my experience of TTY systems, there's generally a prompt of some kind for "command mode", but not in "input mode", Line layout is generally not critical for commands, but may be so for input of program code or data.
    – dave
    May 6, 2022 at 17:35

In the microcomputer realm, it has certainly been common for programs that were expecting e.g. a 3-character input, to output a prompt followed by three underscores and three backspaces. I don't know to what extent such things were done on other machines. A difficulty with such designs on Unix is that unless the console is placed in "raw" mode, it's impossible for an application to ensure that the cursor will alwyas be in the expected place relative to such prompts.

I would be surprised if there weren't some programs for Unix or other "big machine" platforms that would show an indication of what kind of input was expected and then backspace over it, but I don't have particular knowledge of such. On microcomputers, however, such behavior was commonplace and useful.

  • I don't remember seeing it before microcomputers.
    – Leo B.
    May 7, 2022 at 17:52

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