Okay, I have probably confused the issue by telling everything, rather than narrowing it down to a single thing.
What I think I have done is set F9 to do the same thing as the "Windows Key", that thing with a Windows logo on it next to the left alt key.
The commands I have used to do this are these:
xmodmap -e "keycode 133 = grave"
xmodmap -e "keycode 75 = grave"
So, 133 is the "Windows Key" and 75 is "grave".
But, when I press F9 it produces grave. When I press "Windows Key" it does nothing.
So, I enter the command sequence:
grave ; escape C-t
which sets the prefix to the usual default of ctrl-t.
Now, pressing F9 produces ` and pressing windows key also produces `, so, having ` as the prefix for stump clearly affected "windows key" in a different way from F9. Why is this?
(Incidentally, pressing prefix key twice did not send the grave, it switched me to the previous task.)
(Sorry for not quoting, but AOL web-mail messed it all up)