mapping keys to other keys

[John Russell]

I have these funny extra keys on my laptop keyboard.  I'd like to have
them make my browser go forward and back, which is what it looks like
they were meant to do, but there is no real command for this.

Does anyone know a way to map these key codes, as determined by xev,
to another key code or combination, such as alt-left/right to do what
I described above?

Thanks

John