qmk-firmware/keyboards/boardsource/3x4/3x4.h

#pragma once

#include "quantum.h"
#define LAYOUT( \
    K00, K01, K02, K03,  \
    K10, K11, K12, K13,  \
    K20, K21, K22, K23 \
    ) { \
        {K00, K01, K02, K03}, \
        {K10, K11, K12, K13}, \
        {K20, K21, K22, K23} \
    }