The compose key is a dead key which introduces a multi-key sequence to generate a character.
You can configure the compose key by using setxkbmap e.g. setxkbmap -option compose:rctrl or starting the X server with the -xkboption option, e.g. X -xkboption compose:ralt. The possibilities for the compose key, e.g. ralt, lwin, rwin, menu, lctrl, rctrl, caps, etc. are listed in the xkeyboard-config(7) man page.
Alternatively, the option lv3:ralt_switch_multikey configures Right Alt as 3rd-level shift, Shift + Right Alt  as Compose. Note that this option is on by default in the gb layout.
The standard compose key sequences available are determined by the locale. Refer to the compose key sequence charts for the available compose key sequences for each locale. Note that most locales using a Microsoft codepage character set (e.g. with a LANG environment variable of the form LL_CC.CPnnn) are not currently recognized by Xlib.
See the Compose(5) man page for more details on how the locale is mapped to a compose key sequence file and how to configure custom compose key sequences.
If an IME is configured, it should be started as part of a remote session (See the Section called Remote sessions via XDMCP).
To use an IME with remote clients started from a command line (See the Section called Displaying remote clients), the IME must be started explicitly, e.g.
After configuring ibus with ibus-setup, start the ibus daemon
$ ibus-daemon -drx
and set XMODIFIERS so XIM clients will use ibus
$ export XMODIFIERS="@im=ibus"
Similarly for fcitx, gcim, iiimf, scim, uim, etc.
pressing the keys in that order. Right Alt + Shift is 4th-level shift.