AD.9.1 If DEL Fails to Delete

Every keyboard has a large key, a little ways above the RET or ENTER key, which you normally use outside Emacs to erase the last character that you typed. We call this key the usual erasure key. In Emacs, it is supposed to be equivalent to DEL, and when Emacs is properly configured for your terminal, it translates that key into the character DEL.

When Emacs starts up using a window system, it determines automatically which key should be DEL. In some unusual cases Emacs gets the wrong information from the system. If the usual erasure key deletes forwards instead of backwards, that is probably what happened--Emacs ought to be treating the DELETE key as DEL, but it isn't.

With a window system, if the usual erasure key is labeled BACKSPACE and there is a DELETE key elsewhere, but the DELETE key deletes backward instead of forward, that too suggests Emacs got the wrong information--but in the opposite sense. It ought to be treating the BACKSPACE key as DEL, and treating DELETE differently, but it isn't.

On a text-only terminal, if you find the usual erasure key prompts for a Help command, like Control-h, instead of deleting a character, it means that key is actually sending the BS character. Emacs ought to be treating BS as DEL, but it isn't.

In all of those cases, the immediate remedy is the same: use the command M-x normal-erase-is-backspace-mode. This toggles between the two modes that Emacs supports for handling DEL, so if Emacs starts in the wrong mode, it should switch to the right mode. On a text-only terminal, if you want to ask for help when BS is treated as DEL, use F1; C-? may also work, if it sends character code 127.

To fix the problem automatically for every Emacs session, you can put one of the following lines into your `.emacs' file (see section AD.7 The Init File, `~/.emacs'). For the first case above, where DELETE deletes forwards instead of backwards, use this line to make DELETE act as DEL (resulting in behavior compatible with Emacs 20 and previous versions):

(normal-erase-is-backspace-mode 0)

For the other two cases, where BACKSPACE ought to act as DEL, use this line:

(normal-erase-is-backspace-mode 1)

Another way to fix the problem for every Emacs session is to customize the variable normal-erase-is-backspace: the value t specifies the mode where BS or BACKSPACE is DEL, and nil specifies the other mode. See section AD.2.2 Easy Customization Interface.

With a window system, it can also happen that the usual erasure key is labeled BACKSPACE, there is a DELETE key elsewhere, and both keys delete forward. This probably means that someone has redefined your BACKSPACE key as a DELETE key. With X, this is typically done with a command to the xmodmap program when you start the server or log in. The most likely motive for this customization was to support old versions of Emacs, so we recommend you simply remove it now.