from the man page

xmodmap - utility for modifying keymaps and pointer button< mappings in X
The xmodmap program is used to edit and display the keyboard modifier map and keymap table that are used by client applications to convert event keycodes into keysyms.
It is usually run from the user's session startup script to configure the keyboard according to personal tastes.

Pasted here largely to illustrate my writeup about keyboards.