This directory is intended for project header files. A header file is a file containing C declarations and macro definitions to be shared between several project source files. You request the use of a header file in your project source file (C, C++, etc) located in `src` folder by including it, with the C preprocessing directive `#include'. ```src/main.c #include "header.h" int main (void) { ... } ``` Including a header file produces the same results as copying the header file into each source file that needs it. Such copying would be time-consuming and error-prone. With a header file, the related declarations appear in only one place. If they need to be changed, they can be changed in one place, and programs that include the header file will automatically use the new version when next recompiled. The header file eliminates the labor of finding and changing all the copies as well as the risk that a failure to find one copy will result in inconsistencies within a program. In C, the convention is to give header files names that end with `.h'. Read more about using header files in official GCC documentation: * Include Syntax * Include Operation * Once-Only Headers * Computed Includes https://gcc.gnu.org/onlinedocs/cpp/Header-Files.html