QMK keymaps are defined inside a C source file. The data structure is an array of arrays. The outer array is a list of layer arrays while the inner layer array is a list of keys. Most keyboards define a `LAYOUT()` macro to help you create this array of arrays.
In QMK, **`const uint16_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS]`** holds multiple **layers** of keymap information in **16 bit** data holding the **action code**. You can define **32 layers** at most.
For trivial key definitions, the higher 8 bits of the **action code** are all 0 and the lower 8 bits holds the USB HID usage code generated by the key as **keycode**.
Respective layers can be validated simultaneously. Layers are indexed with 0 to 31 and higher layer has precedence.
Keymap layer '0' is usually the `default_layer`, with other layers initially off after booting up the firmware, although this can configured differently in `config.h`. It is useful to change `default_layer` when you completely switch a key layout, for example, if you want to switch to Colemak instead of Qwerty.
On the other hand, you can change `layer_state` to overlay the base layer with other layers for features such as navigation keys, function keys (F1-F12), media keys, and/or special actions.
Note that ***higher layers have higher priority within the stack of layers***. The firmware works its way down from the highest active layers to look up keycodes. Once the firmware locates a keycode other than `KC_TRNS` (transparent) on an active layer, it stops searching, and lower layers aren't referenced.
In the above scenario, the non-transparent keys on the higher layer would be usable, but whenever `KC_TRNS` (or equivalent) is defined, the keycode (`KC_A`) on the lower level would be used.
For this example we will walk through an [older version of the default Clueboard 66% keymap](https://github.com/qmk/qmk_firmware/blob/ca01d94005f67ec4fa9528353481faa622d949ae/keyboards/clueboard/keymaps/default/keymap.c). You'll find it helpful to open that file in another browser window so you can look at everything in context.
These are some handy definitions we can use when building our keymap and our custom function. The `GRAVE_MODS` definition will be used later in our custom function, and the following `_BL`, `_FL`, and `_CL` defines make it easier to refer to each of our layers.
Note: You may also find some older keymap files may also have a define(s) for `_______` and/or `XXXXXXX`. These can be used in place for `KC_TRNS` and `KC_NO` respectively, making it easier to see what keys a layer is overriding. These definitions are now unnecessary, as they are included by default.
The main part of this file is the `keymaps[]` definition. This is where you list your layers and the contents of those layers. This part of the file begins with this definition:
After this you'll find the layer definitions. Typically you'll have one or more "base layers" (such as QWERTY, Dvorak, or Colemak) and then you'll layer on top of that one or more "function" layers. Due to the way layers are processed you can't overlay a "lower" layer on top of a "higher" layer.
`keymaps[][MATRIX_ROWS][MATRIX_COLS]` in QMK holds the 16 bit action code (sometimes referred as the quantum keycode) in it. For the keycode representing typical keys, its high byte is 0 and its low byte is the USB HID usage ID for keyboard.
* The layer is defined using the LAYOUT macro, traditionally defined in the keyboard's `.h` file.
* The LAYOUT macro takes a single list of keycodes, but we have written it in the C source using embedded whitespace and newlines to visualize where each key is on the physical device.
* The LAYOUT macro hides and handles the mapping to the hardware's key scan matrix.
* Plain keyboard scancodes are prefixed with KC_, while "special" keys are not.
* The upper left key activates custom function 0 (`F(0)`)
* The "Fn" key is defined with `MO(_FL)`, which moves to the `_FL` layer while that key is being held down.
#### Function Overlay Layer
Our function layer is, from a code point of view, no different from the base layer. Conceptually, however, you will build that layer as an overlay, not a replacement. For many people this distinction does not matter, but as you build more complicated layering setups it matters more and more.
We are actively working to improve these docs. If you have suggestions for how they could be made better please [file an issue](https://github.com/qmk/qmk_firmware/issues/new)!
