Peter's keyboard firmware (QMK)
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Advanced Keycodes

Your keymap can include keycodes that are more advanced than normal, for example keys that switch layers or send modifiers when held, but send regular keycodes when tapped. This page documents the functions that are available to you.

Assigning Custom Names

People often define custom names using #define. For example:


This will allow you to use FN_CAPS and ALT_TAB in your keymap, keeping it more readable.


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.

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.

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.


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.


The Mod-Tap key MT(mod, kc) acts like a modifier when held, and a regular keycode when tapped. In other words, you can have a key that sends Escape when you tap it, but functions as a Control or Shift key when you hold it down.

The modifiers this keycode and OSM() accept are prefixed with MOD_, not KC_:

Modifier Description
MOD_LCTL Left Control
MOD_LSFT Left Shift
MOD_LGUI Left GUI (Windows/Command/Meta key)
MOD_RCTL Right Control
MOD_RSFT Right Shift
MOD_RALT Right Alt (AltGr)
MOD_RGUI Right GUI (Windows/Command/Meta key)
MOD_HYPR Hyper (Left Control, Shift, Alt and GUI)
MOD_MEH Meh (Left Control, Shift, and Alt)

You can combine these by ORing them together like so:


This key would activate Left Control and Left Shift when held, and send Escape when tapped.

For convenience, QMK includes some Mod-Tap shortcuts to make common combinations more compact in your keymap:

Key Aliases Description
LCTL_T(kc) CTL_T(kc) Left Control when held, kc when tapped
LSFT_T(kc) SFT_T(kc) Left Shift when held, kc when tapped
LALT_T(kc) ALT_T(kc) Left Alt when held, kc when tapped
LGUI_T(kc) LCMD_T(kc), LWIN_T(kc), GUI_T(kc), CMD_T(kc), WIN_T(kc) Left GUI when held, kc when tapped
RCTL_T(kc) Right Control when held, kc when tapped
RSFT_T(kc) Right Shift when held, kc when tapped
RALT_T(kc) ALGR_T(kc) Right Alt when held, kc when tapped
RGUI_T(kc) RCMD_T(kc), RWIN_T(kc) Right GUI when held, kc when tapped
SGUI_T(kc) SCMD_T(kc), SWIN_T(kc) Left Shift and GUI when held, kc when tapped
LCA_T(kc) Left Control and Alt when held, kc when tapped
LCAG_T(kc) Left Control, Alt and GUI when held, kc when tapped
RCAG_T(kc) Right Control, Alt and GUI when held, kc when tapped
C_S_T(kc) Left Control and Shift when held, kc when tapped
MEH_T(kc) Left Control, Shift and Alt when held, kc when tapped
HYPR_T(kc) ALL_T(kc) Left Control, Shift, Alt and GUI when held, kc when tapped - more info here


Unfortunately, these keycodes cannot be used in Mod-Taps or Layer-Taps, since any modifiers specified in the keycode are ignored.

Additionally, you may run into issues when using Remote Desktop Connection on Windows. Because these codes send shift very fast, Remote Desktop may miss the codes.

To fix this, open Remote Desktop Connection, click on “Show Options”, open the the “Local Resources” tab. In the keyboard section, change the drop down to “On this Computer”. This will fix the issue, and allow the characters to work correctly.

One Shot Keys

One shot keys are keys that remain active until the next key is pressed, and then are released. This allows you to type keyboard combinations without pressing more than one key at a time. These keys are usually called “Sticky keys” or “Dead keys”.

For example, if you define a key as OSM(MOD_LSFT), you can type a capital A character by first pressing and releasing shift, and then pressing and releasing A. Your computer will see the shift key being held the moment shift is pressed, and it will see the shift key being released immediately after A is released.

One shot keys also work as normal modifiers. If you hold down a one shot key and type other keys, your one shot will be released immediately after you let go of the key.

Additionally, hitting keys five times in a short period will lock that key. This applies for both One Shot Modifiers and One Shot Layers, and is controlled by the ONESHOT_TAP_TOGGLE define.

You can control the behavior of one shot keys by defining these in config.h:

#define ONESHOT_TAP_TOGGLE 5  /* Tapping this number of times holds the key until tapped once again. */
#define ONESHOT_TIMEOUT 5000  /* Time (in ms) before the one shot key is released */
  • OSM(mod) - Momentarily hold down mod. You must use the MOD_* keycodes as shown in Mod Tap, not the KC_* codes.
  • OSL(layer) - momentary switch to layer.

Sometimes, you want to activate a one-shot key as part of a macro or tap dance routine.

For one shot layers, you need to call set_oneshot_layer(LAYER, ONESHOT_START) on key down, and set_oneshot_layer(ONESHOT_PRESSED) on key up. If you want to cancel the oneshot, call reset_oneshot_layer().

For one shot mods, you need to call set_oneshot_mods(MOD) to set it, or clear_oneshot_mods() to cancel it.

!> If you’re having issues with OSM translating over Remote Desktop Connection, this can be fixed by opening the settings, going to the “Local Resources” tap, and in the keyboard section, change the drop down to “On this Computer”. This will fix the issue and allow OSM to function properly over Remote Desktop.


When you’d like to perform custom logic when pressing a one shot key, there are several callbacks you can choose to implement. You could indicate changes in one shot keys by flashing an LED or making a sound, for example.

There is a callback for OSM(mod). It is called whenever the state of any one shot modifier key is changed: when it toggles on, but also when it is toggled off. You can use it like this:

void oneshot_mods_changed_user(uint8_t mods) {
  if (mods & MOD_MASK_SHIFT) {
    println("Oneshot mods SHIFT");
  if (mods & MOD_MASK_CTRL) {
    println("Oneshot mods CTRL");
  if (mods & MOD_MASK_ALT) {
    println("Oneshot mods ALT");
  if (mods & MOD_MASK_GUI) {
    println("Oneshot mods GUI");
  if (!mods) {
    println("Oneshot mods off");

The mods argument contains the active mods after the change, so it reflects the current state.

When you use One Shot Tap Toggle (by adding #define ONESHOT_TAP_TOGGLE 2 in your config.h file), you may lock a modifier key by pressing it the specified amount of times. There’s a callback for that, too:

void oneshot_locked_mods_changed_user(uint8_t mods) {
  if (mods & MOD_MASK_SHIFT) {
    println("Oneshot locked mods SHIFT");
  if (mods & MOD_MASK_CTRL) {
    println("Oneshot locked mods CTRL");
  if (mods & MOD_MASK_ALT) {
    println("Oneshot locked mods ALT");
  if (mods & MOD_MASK_GUI) {
    println("Oneshot locked mods GUI");
  if (!mods) {
    println("Oneshot locked mods off");

Last, there is also a callback for the OSL(layer) one shot key:

void oneshot_layer_changed_user(uint8_t layer) {
  if (layer == 1) {
    println("Oneshot layer 1 on");
  if (!layer) {
    println("Oneshot layer off");

If any one shot layer is switched off, layer will be zero. When you’re looking to do something on any layer change instead of one shot layer changes, layer_state_set_user is a better callback to use.

If you are making your own keyboard, there are also _kb equivalent functions:

void oneshot_locked_mods_changed_kb(uint8_t mods);
void oneshot_mods_changed_kb(uint8_t mods);
void oneshot_layer_changed_kb(uint8_t layer);

As with any callback, be sure to call the _user variant to allow for further customizability.

Tap-Hold Configuration Options

While Tap-Hold options are fantastic, they are not without their issues. We have tried to configure them with reasonal defaults, but that may still cause issues for some people.

These options let you modify the behavior of the Tap-Hold keys.

Permissive Hold

As of PR#1359, there is a new config.h option:


This makes tap and hold keys (like Mod Tap) work better for fast typist, or for high TAPPING_TERM settings.

If you press a Mod Tap key, tap another key (press and release) and then release the Mod Tap key, all within the tapping term, it will output the “tapping” function for both keys.

For Instance:

  • SHFT_T(KC_A) Down
  • KC_X Down
  • KC_X Up
  • SHFT_T(KC_A) Up

Normally, if you do all this within the TAPPING_TERM (default: 200ms) this will be registered as ax by the firmware and host system. With permissive hold enabled, this modifies how this is handled by considering the Mod Tap keys as a Mod if another key is tapped, and would registered as X (SHIFT+x).

?> If you have Ignore Mod Tap Interrupt enabled, as well, this will modify how both work. The regular key has the modifier added if the first key is released first or if both keys are held longer than the TAPPING_TERM.

Ignore Mod Tap Interrupt

To enable this setting, add this to your config.h:


Similar to Permissive Hold, this alters how the firmware processes input for fast typist. If you press a Mod Tap key, press another key, release the Mod Tap key, and then release the normal key, it would normally output the “tapping” function for both keys. This may not be desirable for rolling combo keys.

Setting Ignore Mod Tap Interrupt requires holding both keys for the TAPPING_TERM to trigger the hold function (the mod).

For Instance:

  • SHFT_T(KC_A) Down
  • KC_X Down
  • SHFT_T(KC_A) Up
  • KC_X Up

Normally, this would send X (SHIFT+x). With Ignore Mod Tap Interrupt enabled, holding both keys are required for the TAPPING_TERM to register the hold action. A quick tap will output ax in this case, while a hold on both will still output X (SHIFT+x).

?> Note: This only concerns modifiers and not layer switching keys.

?> If you have Permissive Hold enabled, as well, this will modify how both work. The regular key has the modifier added if the first key is released first or if both keys are held longer than the TAPPING_TERM.

Tapping Force Hold

To enable tapping force hold, add the following to your config.h:


When the user holds a key after tap, this repeats the tapped key rather to hold a modifier key. This allows to use auto repeat for the tapped key.


  • SHFT_T(KC_A) Down
  • SHFT_T(KC_A) Up
  • SHFT_T(KC_A) Down
  • wait more than tapping term…
  • SHFT_T(KC_A) Up

With default settings, a will be sent on the first release, then a will be sent on the second press allowing the computer to trigger its auto repeat function.

With TAPPING_FORCE_HOLD, the second press will be interpreted as a Shift, allowing to use it as a modifier shortly after having used it as a tap.

!> TAPPING_FORCE_HOLD will break anything that uses tapping toggles (Such as the TT layer keycode, and the One Shot Tapping Toggle).

Retro Tapping

To enable retro tapping, add the following to your config.h:


Holding and releasing a dual function key without pressing another key will result in nothing happening. With retro tapping enabled, releasing the key without pressing another will send the original keycode even if it is outside the tapping term.

For instance, holding and releasing LT(2, KC_SPACE) without hitting another key will result in nothing happening. With this enabled, it will send KC_SPACE instead.