peter@hp-s3030:~$ man x
...
KEYBOARDS
The X keyboard model is broken into two layers: server-specific codes (called keycodes) which represent the physical keys, and server-independent symbols (called keysyms) which represent the letters or words that appear on the keys.
Two tables are kept in the server for converting keycodes to keysyms:
modifier list: Some keys (such as Shift, Control, and Caps Lock) are known as modifier and are used to select different symbols that are attached to a single key (such as Shift-a generates a capital A, and Control-l generates a control character ^L). The server keeps a list of keycodes
corresponding to the various modifier keys. Whenever a key is pressed or released, the server generates an event that contains the keycode of the indicated key as well as a mask that specifies which of the modifier keys are currently pressed. Most servers set up this list to initially contain the various shift, control, and shift lock keys on the keyboard.
keymap table: Applications translate event keycodes and modifier masks into keysyms using a keysym table which contains one row for each keycode and one column for various modifier states. This table is initialized by the server to correspond to normal typewriter conventions. The exact semantics of how the table is interpreted to produce keysyms depends on the particular program, libraries, and language input method used, but the following conventions for the first four keysyms in each row are generally adhered to:
The first four elements of the list are split into two groups of keysyms. Group 1 contains the first and second keysyms; Group 2 contains the third and fourth keysyms. Within each group, if the first element is alphabetic and the the second element is the special keysym NoSymbol, then the group is treated as equivalent to a group in which the first element is the lowercase letter and the second element is the uppercase letter.
Switching between groups is controlled by the keysym named MODE SWITCH, by attaching that keysym to some key and attaching that key to any one of the modifiers Mod1 through Mod5. This modifier is called the ``group modifier.'' Group 1 is used when the group modifier is off, and Group 2 is used when the group modifier is on.
Within a group, the modifier state determines which keysym to use. The first keysym is used when the Shift and Lock modifiers are off. The second keysym is used when the Shift modifier is on, when the Lock modifier is on and the second keysym is uppercase alphabetic, or when the Lock modifier is on and is interpreted as ShiftLock. Otherwise, when the Lock modifier is on and is interpreted as CapsLock, the state of the Shift modifier is applied first to select a keysym; but if that keysym is lowercase alphabetic, then the corresponding uppercase keysym is used instead.