[Boost-users] Why something like #ifndef BOOST_MPI_HPP is always capitalized?

Hi, In most of the C++ header files I've seem, a macro like the following is used to guard repetitive inclusion of the file. I'm wondering why it is always in upper case. Why not make it the same case as the file name? What if there are two header files on is in lower case, the other is in upper case? Thanks, Peng #ifndef BOOST_MPI_HPP