qmk_firmware/keyboards/lets_split/lets_split.h

#ifndef LETS_SPLIT_H
#define LETS_SPLIT_H

#include "quantum.h"

#ifdef KEYBOARD_lets_split_rev1
    #include "rev1.h"
#else
    #include "rev2.h"
#endif

// Used to create a keymap using only KC_ prefixed keys
#define KC_KEYMAP( \
    L00, L01, L02, L03, L04, L05, R00, R01, R02, R03, R04, R05, \
    L10, L11, L12, L13, L14, L15, R10, R11, R12, R13, R14, R15, \
    L20, L21, L22, L23, L24, L25, R20, R21, R22, R23, R24, R25, \
    L30, L31, L32, L33, L34, L35, R30, R31, R32, R33, R34, R35 \
    ) \
    KEYMAP( \
        KC_##L00, KC_##L01, KC_##L02, KC_##L03, KC_##L04, KC_##L05, KC_##R00, KC_##R01, KC_##R02, KC_##R03, KC_##R04, KC_##R05, \
        KC_##L10, KC_##L11, KC_##L12, KC_##L13, KC_##L14, KC_##L15, KC_##R10, KC_##R11, KC_##R12, KC_##R13, KC_##R14, KC_##R15, \
        KC_##L20, KC_##L21, KC_##L22, KC_##L23, KC_##L24, KC_##L25, KC_##R20, KC_##R21, KC_##R22, KC_##R23, KC_##R24, KC_##R25, \
        KC_##L30, KC_##L31, KC_##L32, KC_##L33, KC_##L34, KC_##L35, KC_##R30, KC_##R31, KC_##R32, KC_##R33, KC_##R34, KC_##R35 \
    )

#define LAYOUT_ortho_4x12 KEYMAP
#define KC_LAYOUT_ortho_4x12 KC_KEYMAP

#endif