I insert the plus minus symbol ± frequently enough that I want to bind it to a key.

From emacs -Q, I've tried this:

(global-set-key (kbd "C-c m") "±")

However, when I enter C-c m, the minibuffer displays C-u 1-, suggesting it's waiting for me to finish entering a sequence.

I can bind C-c m to other symbols:

(global-set-key (kbd "C-c m") "⇔")

This works as expected: C-c m inserts the double-arrow .

I've tried other keys as well, e.g.

(global-set-key (kbd "C-c a") "±")

This gives the same result: emacs interprets it as C-u 1-.

Is there something special about the plus-minus glyph that makes Emacs intepret it as a prefix argument? Why can't I insert it as I would other characters?

I have solved the immediate problem by defining an abbrev to insert the plus-minus glyph, but I would like to know why the keybindings above don't work.


Inserting the character via (insert ) does work:

(global-set-key (kbd "C-c m") (lambda () (interactive) (insert "±")))
  • Weird. Out of curiosity, try binding the key to a call to insert wrapped in a lambda. – Dan Dec 21 '18 at 15:58
  • done! That does work. – Tyler Dec 21 '18 at 16:15
  • 1
    This looks like a bug, to me. – Drew Dec 21 '18 at 16:47
  • I agree with @Drew. And bugs can be reported with M-x report-emacs-bug. More information is here. – zck Dec 21 '18 at 16:52
  • I submitted Emacs bug #33829 for this. It might not be considered a bug. Probably has to do with what's allowed in the string value for a keyboard macro, and how such a string is interpreted as a macro. – Drew Dec 21 '18 at 16:53

With respect to why you saw what you saw:

As Eli Zaretskii said in a reply to bug report #33829, the string is interpreted as a keyboard macro, and Emacs "interprets ±, which is a single byte with the 8th bit set, as a meta character".

It's interpreted as M-1, which is shown in the echo area as C-u 1-, as the beginning of a numeric prefix arg.

The mistake/gotcha is to expect that all bytes in a keyboard-macro string are interpreted as characters (which are just inserted). This is a byte that is interpreted as a key bound to a command. It just happens to be renderable also as a Unicode character, ±.

In the context of key-binding to a (keyboard-macro) string, as in the example -- especially since that is easily and commonly used to insert a Unicode character (or any char that is not so easy to type), it's easy to forget that keyboard-macro strings are command sequences.

  • Great, thanks. That makes sense. Avoiding the gotcha seems to depend on you knowing when extended ascii stops and unicode starts, or otherwise distinguishing between single and multi-byte characters. In my case, I pulled the symbols from helm-unicode, which doesn't indicate which symbols are multi-byte (nor would I have expected it to, before now). Fun fact: ± is a single byte, but ∓ is multibyte :) – Tyler Dec 21 '18 at 22:35

You can get what you want with

(global-set-key (kbd "C-c m") [177])

or if you prefer

(global-set-key (kbd "C-c m") [?±])

Another solution uses the package key-chord which allows you to assign commands to keys pressed simultaneously. After you have installed it, you can define

(key-chord-define-global "-=" [177])

and then pressing "-" and "=" at the same time does what you want.

Using vectors to represent unicode characters seems to be more reliable than keyboard macros. Emacs calls self-insert-command on the character C when it sees the key [C] where C is the character (the decimal repersentation) of the unicode character you want.

  • Interesting, thanks. I'm curious what the difference between a vector and the equivalent keyboard macro is. There are a few ascii codes that are used to represent modified numbers. ±²³´µ¶·¸¹° are used for M-1 through M-0, but this only effects keyboard macros, not vectors, as your solution demonstrates. – Tyler Dec 21 '18 at 21:14
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    I think what is happening is that M- is still thought of as changing one bit (presumably adding 128) to the ascii code for 1,2 etc. So it is that the keyboard macro "±" is interpreted as "M-1". – Aidan Schofield Dec 21 '18 at 21:31
  • that looks right, and applies to other characters too - the symbol î is ascii 238, or 110 + 128. n is ascii 110, so trying to use the keyboard macro "î" fails too - presumably because Emacs uses that code to represent M-n. – Tyler Dec 21 '18 at 21:42
  • Here's confirmation from Eli Z: debbugs.gnu.org/cgi/bugreport.cgi?bug=33829#11 I guess this is obvious to people who know which characters are single bytes and which are multibytes ;) – Tyler Dec 21 '18 at 21:45

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