Following up on a previous article.
On certain keyboards, there is no right Windows key to set up a “compose” key.
/usr/share/X11/xkb/rules/xorg.lst, you can find the default keys that can be defined as "compose":
Compose key Compose key position compose:ralt Right Alt is Compose. compose:lwin Left Win-key is Compose. compose:rwin Right Win-key is Compose. compose:menu Menu is Compose. compose:rctrl Right Ctrl is Compose. compose:caps Caps Lock is Compose
All set now to edit
EDIT 070820: On Ubuntu, look at the file
/usr/share/X11/locale/iso8859-1/Compose for a complete list of the special characters you can get with the "compose" key (depends on your keyboard layout and locales).