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 KEYMAP() macro to help you create this array of arrays.
For this example we will walk through the [default Clueboard keymap](https://github.com/qmk/qmk_firmware/blob/master/keyboards/clueboard/keymaps/default/keymap.c). There are 3 main sections of a `keymap.c` file you'll want to concern yourself with:
// Each layer gets a name for readability, which is then used in the keymap matrix below.
// The underscores don't mean anything - you can have a layer called STUFF or any other name.
// Layer names don't all need to be of the same length, obviously, and you can also skip them
// entirely and just use numbers.
#define _BL 0
#define _FL 1
#define _CL 2
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. The `_______` define makes it easier to see what keys a layer is overriding, while the `_BL`, `_FL`, and `_CL` defines make it easier to refer to each of our layers.
### Layers and Keymaps
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 a list of KEYMAP() macros. A KEYMAP() is simply a list of keys to define a single layer. 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.
* From a C source point of view it's only a single array, but we have embedded whitespace to more easily visualize where each key is on the physical device.
* 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 have used our `_______` definition to turn `KC_TRNS` into `_______`. This makes it easier to spot the keys that have changed on this layer.
* While in this layer if you press one of the `_______` keys it will activate the key in the next lowest active layer.
### Custom Functions
At the bottom of the file we've defined a single custom function. This function defines a key that sends `KC_ESC` when pressed without modifiers and `KC_GRAVE` when modifiers are held. There are a couple pieces that need to be in place for this to work, and we will go over both of them.
#### `fn_actions[]`
We define the `fn_actions[]` array to point to custom functions. `F(N)` in a keymap will call element N of that array. In the Clueboard's case that definition looks like this:
In this case we've instructed QMK to call the `ACTION_FUNCTION` callback, which we will define in the next section.
#### `action_function()`
To actually handle the keypress event we define an `action_function()`. This function will be called when the key is pressed, and then again when the key is released. We have to handle both situations within our code, as well as determining whether to send/release `KC_ESC` or `KC_GRAVE`.
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)!
