qmk_firmware/docs/mod_tap.md

Mod-Tap

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_LALT	Left Alt
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:

MT(MOD_LCTL | MOD_LSFT, KC_ESC)

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)	LOPT_T(kc), ALT_T(kc), OPT_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)	ROPT_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
LSG_T(kc)	SGUI_T(kc), SCMD_T(kc), SWIN_T(kc)	Left Shift and GUI when held, kc when tapped
LAG_T(kc)		Left Alt and GUI when held, kc when tapped
RSG_T(kc)		Right Shift and GUI when held, kc when tapped
RAG_T(kc)		Right Alt and GUI when held, kc when tapped
LCA_T(kc)		Left Control and Alt when held, kc when tapped
LSA_T(kc)		Left Shift and Alt when held, kc when tapped
RSA_T(kc)	SAGR_T(kc)	Right Shift and Right Alt (AltGr) when held, kc when tapped
RCS_T(kc)		Right Control and Right Shift 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

Caveats

Currently, the kc argument of MT() is limited to the Basic Keycode set, meaning you can't use keycodes like LCTL(), KC_TILD, or anything greater than 0xFF. This is because QMK uses 16-bit keycodes, of which 3 bits are used for the function identifier, 1 bit for selecting right or left mods, and 4 bits to tell which mods are used, leaving only 8 bits for the keycode. Additionally, if at least one right-handed modifier is specified in a Mod-Tap, it will cause all modifiers specified to become right-handed, so it is not possible to mix and match the two - for example, Left Control and Right Shift would become Right Control and Right Shift.

Expanding this would be complicated, at best. Moving to a 32-bit keycode would solve a lot of this, but would double the amount of space that the keymap matrix uses. And it could potentially cause issues, too. If you need to apply modifiers to your tapped keycode, Tap Dance can be used to accomplish this.

You may also run into issues when using Remote Desktop Connection on Windows. Because these keycodes send key events faster than a human, Remote Desktop could miss them. To fix this, open Remote Desktop Connection, click on "Show Options", open the "Local Resources" tab, and in the keyboard section, change the drop down to "On this Computer". This will fix the issue, and allow the characters to work correctly. It can also be mitigated by increasing TAP_CODE_DELAY.

Intercepting Mod-Taps

Changing tap function

The basic keycode limitation with Mod-Tap can be worked around by intercepting it in process_record_user. For example, shifted keycode KC_DQUO cannot be used with MT() because it is a 16-bit keycode alias of LSFT(KC_QUOT). Modifiers on KC_DQUO will be masked by MT(). But the following custom code can be used to intercept the "tap" function to manually send KC_DQUO:

bool process_record_user(uint16_t keycode, keyrecord_t *record) {
    switch (keycode) {
        case LCTL_T(KC_DQUO):
            if (record->tap.count && record->event.pressed) {
                tap_code16(KC_DQUO); // Send KC_DQUO on tap
                return false;        // Return false to ignore further processing of key
            }
            break;
    }
    return true;
}

Changing hold function

Likewise, similar custom code can also be used to intercept the hold function to send custom user key code. The following example uses LT(0, kc) (layer-tap key with no practical use because layer 0 is always active) to add cut, copy and paste function to X,C and V keys when they are held down:

bool process_record_user(uint16_t keycode, keyrecord_t *record) {
    switch (keycode) {
        case LT(0,KC_X):
            if (!record->tap.count && record->event.pressed) {
                tap_code16(C(KC_X)); // Intercept hold function to send Ctrl-X
                return false;
            }
            return true;             // Return true for normal processing of tap keycode
        case LT(0,KC_C):
            if (!record->tap.count && record->event.pressed) {
                tap_code16(C(KC_C)); // Intercept hold function to send Ctrl-C
                return false;
            }
            return true;             // Return true for normal processing of tap keycode
        case LT(0,KC_V):
            if (!record->tap.count && record->event.pressed) {
                tap_code16(C(KC_V)); // Intercept hold function to send Ctrl-V
                return false;
            }
            return true;             // Return true for normal processing of tap keycode
    }
    return true;
}

Enabling IGNORE_MOD_TAP_INTERRUPT is recommended when using Mod-Tap on alphanumeric keys to avoid hold function taking precendence when the next key is pressed quickly. See Ignore Mod Tap Interrupt for more details.

Changing both tap and hold

This last example implements custom tap and hold function with LT(0,KC_NO) to create a single copy-on-tap, paste-on-hold key:

bool process_record_user(uint16_t keycode, keyrecord_t *record) {
    switch (keycode) {
        case LT(0,KC_NO):
            if (record->tap.count && record->event.pressed) {
                tap_code16(C(KC_C)); // Intercept tap function to send Ctrl-C
            } else if (record->event.pressed) {
                tap_code16(C(KC_V)); // Intercept hold function to send Ctrl-V
            }
            return false;
    }
    return true;
}

Other Resources

See the Tap-Hold Configuration Options for additional flags that tweak Mod-Tap behavior.