Compose Key is a special key on your keyboard that allows you to type special characters like →, °, €, ä, ™, ®, ¿ and many more. It’s very useful if you’re writing in any foreign language that uses special characters and don’t want to mangle your keyboard settings all the time.

Examples

• Compose + - + > will produce a → character
• Compose + o + o will produce a ° character
• Compose + c + = will produce a € character
• Compose + a + " will produce a ä character
• Compose + t + m will produce a ™ character
• Compose + r + o will produce a ® character
• Compose + ? + ? will produce a ¿ character

As you can see in the examples above, the combinations are very thought out and easy to remember. Most of the time you don’t need to know them by heart. You just have to think what two (or more) keys from the standard key set combined will give you the character that you want.

How to enable Compose Key?

As I use Gnome 3, I’ll describe how to set it up in this environment. But don’t worry, it can be enabled in pretty much every WM/DE.

So, at first you have to go to the Settings and then Keyboard:

Then you have to click the Compose Key option in the Special Character Entry section. There you can turn it on and choose which key you want to use as a Compose Key.

As you can see on the screenshot, I use the Caps Lock key, but choose whatever you like.

Lists of possible combinations

These two lists are the best that I found in the internet:

1. GTKComposeTable - rather short, very readable list of most useful combinations.
2. libX11 documentation - very long, hard to read but very complete list of all possible combinations.