[ Thanks to linuxaria for this link.
“First, I must say that Linux (Unix) uses two
approaches to configure the keyboard layout (they are both
independent of each other): XKB and Xmodmap. XKB is an extension of
X, many people say that it is better, but too robust and perhaps
less understandable by beginners. Xmodmap is one of the oldest ways
how to configure your keyboard layouts – a little easier
approach, especially good for experimentation, but no only that.
“XKB, too, allows a number of easy solutions, but if we look at
hundreds of Linux distributions and Window Managers including BSD,
Xmodmap solution basically works the same way everywhere –
whether on PC-BSD, SuSE Linux, Debian Linux, Slackware Linux,