RPi Compose key
A Compose key is a keyboard key that modfies the effect of keys pressed after it. It allows the entry of accented characters or special symbols on a regular keyboard. For example, Compose followed by " and u produces the character lower case u umlaut, ü, while Compose followed by : and - produces the division operator, ÷.
The Compose key is defined in the configure_keyboard section of raspi-config. It is often chosen to be one of the modifier keys to the right of the space bar (such as Alt Gr, Windows Logo, or Ctrl). If you don't think you need one, you can select “None”.
The function of the Compose key is limited to accented characters on the text console, but has full function under X/LXDE. Here is a list of common Compose key combinations.