File inclusion makes it easy to handle collections of #defines and declarations (among other things). Any source line of the form
#include "filename"
or
#include <filename>
is replaced by the contents of the file filename. If the filename is quoted, searching for the file typically begins where the source program was found; if it is not found there, or if the name is enclosed in < and >, searching follows an implementation-defined rule to find the file. An included file may itself contain #include lines.