C++ #ifndef for include files, why is all caps used for the header file?

Question

I wonder why the name after the #ifndef directive is always all caps and don\'t seem to match the name of the actual header file? What are the rules surrounding

Answer 1

These are preprocessor symbols and have no such rules. (as long as they match the #defines in the headers)

However, convention is to use all-caps for preprocessor symbols.