For instance, I can enter a pound  or yen  symbol, but not a Euro symbol; a copyright  but not trademark, and so on.  All the characters seem to be defined in the system compose file (/usr/share/X11/locale/en_US.UTF-8/Compose), and I can see nothing different about the ones that work vs ones that don't.I'm running OpenSuSE 13.1 with fvm2 window manager.  IIRC, setup is just default US English.  Working vs non-working seems to be the same in xterm, or in apps like the browser I'm using now.The output of running locale; export | grep IM_MODULE is simply to run locale, and then to state export: Command not found. The output of locale isLANG=en_US.UTF-8LC_CTYPE=fr_CHLC_NUMERIC=en_US.UTF-8LC_TIME=en_US.UTF-8LC_COLLATE=en_US.UTF-8LC_MONETARY=en_US.UTF-8LC_MESSAGES=en_US.UTF-8LC_PAPER=en_US.UTF-8LC_NAME=en_US.UTF-8LC_ADDRESS=en_US.UTF-8LC_TELEPHONE=en_US.UTF-8LC_MEASUREMENT=en_US.UTF-8LC_IDENTIFICATION=en_US.UTF-8LC_ALL=(The LC_CTYPE is to make emails in French readable.)  There doesn't seem to be an IM_MODULE variable in the environment.  Doing env | grep IM givesQT_IM_MODULE=ximXMODIFIERS=@im=localXDG_RUNTIME_DIR=/run/user/1000GTK_IM_MODULE=cedillaQT_IM_SWITCHER=imsw-multiBut those don't seem relevant, as I'm not trying it with QT or GTK applications.  (AFAIK, I don't know for sure what the browsers use under the hood.)
Why some compose keys don't work?
compose key