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Switching and Toggling Layers

These functions allow you to activate layers in various ways. Note that layers are not generally independent layouts -- multiple layers can be activated at once, and it's typical for layers to use KC_TRNS to allow keypresses to pass through to lower layers. For a detailed explanation of layers, see Keymap Overview. When using momentary layer switching with MO(), LM(), TT(), or LT(), make sure to leave the key on the above layers transparent or it may not work as intended.

DF(layer) - switches the default layer. The default layer is the always-active base layer that other layers stack on top of. See below for more about the default layer. This might be used to switch from QWERTY to Dvorak layout. (Note that this is a temporary switch that only persists until the keyboard loses power. To modify the default layer in a persistent way requires deeper customization, such as calling the set_single_persistent_default_layer function inside of process_record_user.)
MO(layer) - momentarily activates layer. As soon as you let go of the key, the layer is deactivated.
LM(layer, mod) - Momentarily activates layer (like MO), but with modifier(s) mod active. Only supports layers 0-15 and the left modifiers: MOD_LCTL, MOD_LSFT, MOD_LALT, MOD_LGUI (note the use of MOD_ constants instead of KC_). These modifiers can be combined using bitwise OR, e.g. LM(_RAISE, MOD_LCTL | MOD_LALT).
LT(layer, kc) - momentarily activates layer when held, and sends kc when tapped. Only supports layers 0-15.
OSL(layer) - momentarily activates layer until the next key is pressed. See One Shot Keys for details and additional functionality.
TG(layer) - toggles layer, activating it if it's inactive and vice versa
TO(layer) - activates layer and de-activates all other layers (except your default layer). This function is special, because instead of just adding/removing one layer to your active layer stack, it will completely replace your current active layers, uniquely allowing you to replace higher layers with a lower one. This is activated on keydown (as soon as the key is pressed).
TT(layer) - Layer Tap-Toggle. If you hold the key down, layer is activated, and then is de-activated when you let go (like MO). If you repeatedly tap it, the layer will be toggled on or off (like TG). It needs 5 taps by default, but you can change this by defining TAPPING_TOGGLE -- for example, #define TAPPING_TOGGLE 2 to toggle on just two taps.

Caveats

Currently, LT() and MT() are limited to the Basic Keycode set, meaning you can't use keycodes like LCTL(), KC_TILD, or anything greater than 0xFF. Modifiers specified as part of a Layer Tap or Mod Tap's keycode will be ignored. If you need to apply modifiers to your tapped keycode, Tap Dance can be used to accomplish this.

Additionally, if at least one right-handed modifier is specified in a Mod Tap or Layer Tap, it will cause all modifiers specified to become right-handed, so it is not possible to mix and match the two.

Working with Layers

Care must be taken when switching layers, it's possible to lock yourself into a layer with no way to deactivate that layer (without unplugging your keyboard.) We've created some guidelines to help users avoid the most common problems.

Beginners

If you are just getting started with QMK you will want to keep everything simple. Follow these guidelines when setting up your layers:

Setup layer 0 as your default, "base" layer. This is your normal typing layer, and could be whatever layout you want (qwerty, dvorak, colemak, etc.). It's important to set this as the lowest layer since it will typically have most or all of the keyboard's keys defined, so would block other layers from having any effect if it were above them (i.e., had a higher layer number).
Arrange your layers in a "tree" layout, with layer 0 as the root. Do not try to enter the same layer from more than one other layer.
In a layer's keymap, only reference higher-numbered layers. Because layers are processed from the highest-numbered (topmost) active layer down, modifying the state of lower layers can be tricky and error-prone.

Intermediate Users

Sometimes you need more than one base layer. For example, if you want to switch between QWERTY and Dvorak, switch between layouts for different countries, or switch your layout for different videogames. Your base layers should always be the lowest numbered layers. When you have multiple base layers you should always treat them as mutually exclusive. When one base layer is on the others are off.

Advanced Users

Once you have a good feel for how layers work and what you can do, you can get more creative. The rules listed in the beginner section will help you be successful by avoiding some of the tricker details but they can be constraining, especially for ultra-compact keyboard users. Understanding how layers work will allow you to use them in more advanced ways.

Layers stack on top of each other in numerical order. When determining what a keypress does, QMK scans the layers from the top down, stopping when it reaches the first active layer that is not set to KC_TRNS. As a result if you activate a layer that is numerically lower than your current layer, and your current layer (or another layer that is active and higher than your target layer) has something other than KC_TRNS, that is the key that will be sent, not the key on the layer you just activated. This is the cause of most people's "why doesn't my layer get switched" problem.

Sometimes, you might want to switch between layers in a macro or as part of a tap dance routine. layer_on activates a layer, and layer_off deactivates it. More layer-related functions can be found in action_layer.h.

Modifier Keys

These allow you to combine a modifier with a keycode. When pressed, the keydown event for the modifier, then kc will be sent. On release, the keyup event for kc, then the modifier will be sent.

Key	Aliases	Description
`LCTL(kc)`	`C(kc)`	Hold Left Control and press `kc`
`LSFT(kc)`	`S(kc)`	Hold Left Shift and press `kc`
`LALT(kc)`	`A(kc)`	Hold Left Alt and press `kc`
`LGUI(kc)`	`G(kc)`, `LCMD(kc)`, `LWIN(kc)`	Hold Left GUI and press `kc`
`RCTL(kc)`		Hold Right Control and press `kc`
`RSFT(kc)`		Hold Right Shift and press `kc`
`RALT(kc)`	`ALGR(kc)`	Hold Right Alt and press `kc`
`RGUI(kc)`	`RCMD(kc)`, `LWIN(kc)`	Hold Right GUI and press `kc`
`SGUI(kc)`	`SCMD(kc)`, `SWIN(kc)`	Hold Left Shift and GUI and press `kc`
`LCA(kc)`		Hold Left Control and Alt and press `kc`
`LCAG(kc)`		Hold Left Control, Alt and GUI and press `kc`
`MEH(kc)`		Hold Left Control, Shift and Alt and press `kc`
`HYPR(kc)`		Hold Left Control, Shift, Alt and GUI and press `kc`

You can also chain them, for example LCTL(LALT(KC_DEL)) makes a key that sends Control+Alt+Delete with a single keypress.

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