I don't work in C or C++ but I'm inferring from the context and other comments that it's actually because .c (lower case) is used to indicate C language code an

I don't work in C or C++ but I'm inferring from the context and other comments that it's actually because .c (lower case) is used to indicate C language code and .C (upper case) means C++ code. This then fails on windows because the OS is not case-sensitive wrt filenames. So to make the implied answer to the question explicit: no, it's because someone who knew the difference between C and C++ sought to distinguish between their source code files using a filename extension convention that becomes invisible on a different OS.