It is possible to install custom [ XKB
](https://en.wikipedia.org/wiki/X_keyboard_extension) keyboard layouts
using the option `services.xserver.xkb.extraLayouts`.

As a first example, we are going to create a layout based on the basic
US layout, with an additional layer to type some greek symbols by
pressing the right-alt key.

Create a file called `us-greek` with the following content (under a
directory called `symbols`; it's an XKB peculiarity that will help with
testing):

```
xkb_symbols "us-greek"
{
  include "us(basic)"            // includes the base US keys
  include "level3(ralt_switch)"  // configures right alt as a third level switch

  key <LatA> { [ a, A, Greek_alpha ] };
  key <LatB> { [ b, B, Greek_beta  ] };
  key <LatG> { [ g, G, Greek_gamma ] };
  key <LatD> { [ d, D, Greek_delta ] };
  key <LatZ> { [ z, Z, Greek_zeta  ] };
};
```

A minimal layout specification must include the following:

```nix
{
  services.xserver.xkb.extraLayouts.us-greek = {
    description = "US layout with alt-gr greek";
    languages   = [ "eng" ];
    symbolsFile = /yourpath/symbols/us-greek;
  };
}
```

::: {.note}
The name (after `extraLayouts.`) should match the one given to the
`xkb_symbols` block.
:::

Applying this customization requires rebuilding several packages, and a
broken XKB file can lead to the X session crashing at login. Therefore,
you're strongly advised to **test your layout before applying it**:

```ShellSession
$ nix-shell -p xorg.xkbcomp
$ setxkbmap -I/yourpath us-greek -print | xkbcomp -I/yourpath - $DISPLAY
```

You can inspect the predefined XKB files for examples:

```ShellSession
$ echo "$(nix-build --no-out-link '<nixpkgs>' -A xorg.xkeyboardconfig)/etc/X11/xkb/"
```

Once the configuration is applied, and you did a logout/login cycle, the
layout should be ready to use. You can try it by e.g. running
`setxkbmap us-greek` and then type `<alt>+a` (it may not get applied in
your terminal straight away). To change the default, the usual
`services.xserver.xkb.layout` option can still be used.