qmk_firmware/docs/feature_tri_layer.md

Tri Layers :id=tri-layers

This enables support for the OLKB style "Tri Layer" keycodes. These function similar to the MO (momentary) function key, but if both the "Lower" and "Upper" keys are pressed, it activates a third "Adjust" layer. To enable this functionality, add this line to your rules.mk:

TRI_LAYER_ENABLE = yes

Note that the "upper", "lower" and "adjust" names don't have a particular significance, they are just used to identify and clarify the behavior. Layers are processed from highest numeric value to lowest, however the values are not required to be consecutive.

For a detailed explanation of how the layer stack works, check out Keymap Overview.

Keycodes :id=keycodes

Keycode	Alias	Description
QK_TRI_LAYER_LOWER	TL_LOWR	Momentarily enables the "lower" layer. Enables the "adjust" layer if the "upper" layer is also enabled"
QK_TRI_LAYER_UPPER	TL_UPPR	Momentarily enables the "upper" layer. Enables the "adjust" layer if the "lower" layer is also enabled"

Configuration

To change the default values for the layers, you can change these defines, in your config.h

Config name	Default	Description
TRI_LAYER_LOWER_LAYER	1	Sets the default for the "lower" layer.
TRI_LAYER_UPPER_LAYER	2	Sets the default for the "upper" layer.
TRI_LAYER_ADJUST_LAYER	3	Sets the default for the "adjust" layer.

Eg, if you wanted to set the "Adjust" layer to be layer 5, you'd add this to your config.h:

#define TRI_LAYER_ADJUST_LAYER 5

Functions

Function name	Description
set_tri_layer_lower_layer(layer)	Changes the "lower" layer*.
set_tri_layer_upper_layer(layer)	Changes the "upper" layer*.
set_tri_layer_adjust_layer(layer)	Changes the "adjust" layer*.
set_tri_layer_layers(lower, upper, adjust)	Sets the "lower", "upper" and "adjust" layers*.
get_tri_layer_lower_layer()	Gets the current "lower" layer.
get_tri_layer_upper_layer()	Gets the current "upper" layer.
get_tri_layer_adjust_layer()	Gets the current "adjust" layer.

!> Note: these settings are not persistent, and will be reset to the default on power loss or power cycling of the controller.