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_`:
|`HYPR_T(kc)`|`ALL_T(kc)` |Left Control, Shift, Alt and GUI when held, `kc` when tapped - more info [here](https://brettterpstra.com/2012/12/08/a-useful-caps-lock-key/)|
Currently, the `kc` argument of `MT()` is limited to the [Basic Keycode set](keycodes_basic.md), 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](feature_tap_dance.md#example-5-using-tap-dance-for-advanced-mod-tap-and-layer-tap-keys) 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 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`](config_options.md#behaviors-that-can-be-configured).
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`:
return false; // Return false to ignore further processing of key
}
break;
}
return true;
}
```
### Changing hold function
Likewise, the same 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:
return true; // Return true for normal processing of tap keycode
} else if (record->event.pressed) {
tap_code16(C(KC_X)); // Intercept hold function to send Ctrl-X
}
return false;
case LT(0,KC_C):
if (record->tap.count && record->event.pressed) {
return true; // Return true for normal processing of tap keycode
} else if (record->event.pressed) {
tap_code16(C(KC_C)); // Intercept hold function to send Ctrl-C
}
return false;
case LT(0,KC_V):
if (record->tap.count && record->event.pressed) {
return true; // Return true for normal processing of tap keycode
} else if (record->event.pressed) {
tap_code16(C(KC_V)); // Intercept hold function to send Ctrl-V
}
return false;
}
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](tap_hold.md#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:
