Setting up a real Compose key on Mac OS X

The Compose key is my method of choice for international character input. It lets me type characters as diverse as é Â ẃ ṗ § … « ¿ ¥ ¹ ½ © × using simple and intuitive key combinations.

How intuitive exactly? Let’s see:

  • Compose + C + , gives me Ç.
  • Compose + l + / gives me ł.
  • Compose + < + < gives me «.
  • Compose + - + > gives me →.
  • Compose + < + 3 gives me ♥.
  • Compose + C + C + C + P gives me ☭ (no kidding).

You can of course set up your own rules. This shit is so powerful that I cannot imagine I could ever use any other input method.

So, one would think that with all its glorious Unix heritage, Mac OS X would let you get the most out of your keyboard like the good old X11 system does. Well it turns out it’s possible, but not straightforward.

Fortunately, other people already did all the work. I will just indicate how to put their stuff together.

Step 1: choose a Compose key

Choose the Compose key so that it is easily accessible but does not prevent you from doing anything you ordinarily do. Fortunately, modern keyboards come with more and more idiotic and useless keys.

I use the Right Alt key as my Compose key. I already have a Left Alt key so the right one is a bit useless to me. And it somehow matches the position of the Compose key on old Sun keyboards.

That would be Right Option on a Mac keyboard. I recommend that.

Step 2: remap the Compose key

The problem is that the Mac OS X keyboard preferences:

  • do not let you differentiate between Left and Right Option keys
  • only let you remap modifier keys to another modifier key (or to nothing)

Fortunately, there is KeyRemap4MacBook that lets you do very low level things with your keyboard. Install it.

We will now remap our compose key to something that the next layer will understand. I chose Shift-Control-F13 for that. It is very unlikely you will need that key combination.

In the file ~/Library/Application Support/KeyRemap4MacBook/private.xml put the following:

<?xml version="1.0"?>
    <name>Send Shift-Ctrl-F13 for Right Option</name>
    <autogen>--KeyToKey-- KeyCode::OPTION_R,
                          KeyCode::F13, ModifierFlag::SHIFT_L
                                      | ModifierFlag::CONTROL_L

Finally, from the System Preferences, open the KeyRemap4MacBook settings and click on the ReloadXML button:

The new option should appear. Activate it:

Step 3: create compose bindings

The last step is the creation of the actual bindings. I chose to import the rules from /usr/share/X11/locale/en_US.UTF-8/Compose on my Debian system.

Bob Kåres wrote a script that lets you convert X11 compose rules into Cocoa key bindings.

Either convert a Compose file of your own using Bob’s script, or download my DefaultKeyBinding.dict. Save it in ~/Library/KeyBindings/DefaultKeyBinding.dict.

Be careful: by default Bob’s script uses F13 instead of Shift-Ctrl-F13 so in DefaultKeyBinding.dict you need to change:




If for some reason you decided to go for another combination, check out this article by Xah Lee to find out the proper syntax.

Step 4: restart all applications

And that’s it! Your Mac OS X system is now slightly more usable.