Editing ASCII (section)

====Delete vs backspace====
The Teletype could not move its typehead backwards, so it did not have a key on its keyboard to send a BS (backspace). Instead, there was a key marked {{keypress|RUB OUT}} that sent code 127 (DEL). The purpose of this key was to erase mistakes in a manually-input paper tape: the operator had to push a button on the tape punch to back it up, then type the rubout, which punched all holes and replaced the mistake with a character that was intended to be ignored.<ref>{{cite mailing list |url=http://lists.gnu.org/archive/html/help-gnu-emacs/2014-05/msg00448.html |title=Re: editor and word processor history (was: Re: RTF for emacs) |author=Barry Margolin |mailing-list=help-gnu-emacs |date=May 29, 2014 |access-date=July 11, 2014 |archive-url=https://web.archive.org/web/20140714133149/http://lists.gnu.org/archive/html/help-gnu-emacs/2014-05/msg00448.html |archive-date=July 14, 2014 |url-status=live }}</ref> Teletypes were commonly used with the less-expensive computers from [[Digital Equipment Corporation]] (DEC); these systems had to use what keys were available, and thus the DEL character was assigned to erase the previous character.<ref name="pdp-6-monitor-manual">{{cite web |url=http://bitsavers.trailing-edge.com/pdf/dec/pdp6/DEC-6-0-EX-SYS-UM-IP-PRE00_Multiprogramming_System_Manual_1965.pdf |title=PDP-6 Multiprogramming System Manual |page=43 |publisher=[[Digital Equipment Corporation]] (DEC) |date=1965 |access-date=July 10, 2014 |archive-url=https://web.archive.org/web/20140714140253/http://bitsavers.trailing-edge.com/pdf/dec/pdp6/DEC-6-0-EX-SYS-UM-IP-PRE00_Multiprogramming_System_Manual_1965.pdf |archive-date=July 14, 2014 |url-status=live }}</ref><ref name="pdp-10-monitor-manual">{{cite web |url=http://bitsavers.org/pdf/dec/pdp10/1970_PDP-10_Ref/1970PDP10Ref_Part3.pdf |title=PDP-10 Reference Handbook, Book 3, Communicating with the Monitor |at=p. 5-5 |publisher=[[Digital Equipment Corporation]] (DEC) |date=1969 |access-date=July 10, 2014 |archive-url=https://web.archive.org/web/20111115083418/http://www.bitsavers.org/pdf/dec/pdp10/1970_PDP-10_Ref/1970PDP10Ref_Part3.pdf |archive-date=November 15, 2011 |url-status=live }}</ref> Because of this, DEC video terminals (by default) sent the DEL character for the key marked "Backspace" while the separate key marked "Delete" sent an [[escape sequence]]; many other competing terminals sent a BS character for the backspace key.

The early Unix tty drivers, unlike some modern implementations, allowed only one character to be set to erase the previous character in canonical input processing (where a very simple line editor is available); this could be set to BS ''or'' DEL, but not both, resulting in recurring situations of ambiguity where users had to decide depending on what terminal they were using ([[Shell (computing)|shells]] that allow line editing, such as [[KornShell|ksh]], [[Bash (Unix shell)|bash]], and [[Z shell|zsh]], understand both). The assumption that no key sent a BS character allowed Ctrl+H to be used for other purposes, such as the "help" prefix command in [[GNU Emacs]].<ref>{{cite web|url=https://www.gnu.org/software/emacs/manual/html_node/emacs/Help.html|title=Help - GNU Emacs Manual|access-date=July 11, 2018|archive-url=https://web.archive.org/web/20180711223750/https://www.gnu.org/software/emacs/manual/html_node/emacs/Help.html|archive-date=July 11, 2018|url-status=live}}</ref>