How the -I DIR option alters the behave of the preprocessor?

[Carlos Eduardo Rodrigues Diogenes]

Hi,

I was studying the code of gnopernicus and I saw that in one of the 
files have the following line:

#include "glib.h"

but the file glib.h is not in the currenty directory to be around "". 
Why that was not write like #include <glib.h>

The -I DIR option alter the way that the preprocessor looks for include 
files declared with ""? This is not clear for me looking only at the 
documentation.

Thanks,
Carlos.