Your keyboard layout is defined by a configuration file called a keymap, which controls what characters appear when you press keys.

They are used to change which key is mapped to which character add characters that are not on your keyboard, or not defined by the base set of characters.

Keymaps are stored in:

/system/keymaps/

For example:

/system/keymaps/en-GB.keymap

A keymap can be loaded at runtime by the KEYMAP Keyword.

Format

Each line is one of:

a key mapping
a symbol definition
a comment (# ...)

Key mappings

&SCANCODE normal shifted

This defines what a key produces:

normal is the character without Shift
shifted is the character with Shift

Example:

&02 1 !

&04 3 &A3

This makes the 3 key produce £ when Shift is held.

Symbol definitions

SYM &XX &b0 &b1 &b2 &b3 &b4 &b5 &b6 &b7

Defines how a character looks on screen.

&XX is the character number (the & means the number is written in hex)
the eight values that follow are the 8 rows of the character
each value is a number from 0 to 255
each bit in that number controls one pixel (1 = on, 0 = off)
the character is 8 pixels high and 8 pixels wide

Example (£):

SYM &A3 &1C &36 &30 &7C &30 &30 &7E &00

This defines the shape used whenever character &A3 is printed.

How to read a row

Take this value:

&1C

In binary, this is:

00011100

This means:

...###..

Each # is a lit pixel, each . is empty.

How it is used

If a key is mapped to a character number, and that character has a symbol definition, that symbol will be shown when the key is pressed.

Example:

&04 3 &A3

SYM &A3 &1C &36 &30 &7C &30 &30 &7E &00

Pressing Shift+3 will produce £.

Notes

symbol definitions only affect how a character looks
redefining a character replaces any previous definition
characters without a definition use the default font
keymaps override the built-in layout
only keys that are listed are changed
duplicate entries are ignored