|Names||Compose, Compose Character|
|X11||Multi_key (keycode 135)|
|USB||Keyboard Application (7:0x65)|
The Compose key or Compose Character key is a dead key used for "composing" special characters by combining existing ones found on the keyboard.
It sometimes has a lamp that tells when the keyboard is in Compose mode.
The Compose key is first pressed, followed by the key sequence for the character. The keys are usually two.
For example Compose, A, E pressed in sequence produces the ligature Æ.
Gnome 2 / Mate
In the menu select System→Preferences→Hardware→Keyboard.
Then select the Layouts tab and click 'Options...
Then click 'Position of Compose key and choose which key that should be the Compose key.
A third-party utility has to be used: