Using #include

Theory

#include in C has two forms.

Quoting the first edition of K&R.

#include "filename"	Causes the replacement of that line by the entire contents of filename. The named file is searched for first in the directory of the original source file, and then in a sequence of standard places.
#include <filename>	Searches the standard places, and not the directory of the source file.

9.1.2 C Header Files (was 7.1.2 in the printed book) of The Goat Book (a.k.a. Autoconf, Automake, and Libtool) advises:

If a header is designed to be installed, it must #include other installed project headers from the local tree using angle-brackets.

Practice

.h files

Header files, whether or not they're currently meant to be installed, use the form '#include <libfo/filename>' for all libfo headers. For example, from the current fo-layout_master_set.h:

#include <libfo/fo-utils.h>
#include <libfo/property/fo-property.h>

.c files

C source code files use the form '#include filename"' for all libfo headers, i.e., without the leading 'libfo/', since using 'libfo/' is verbose and does not add any extra information.

Including header files in a different subdirectory of the libfo source code directory need to be prefixed by the subdirectory name, for example:

#include "fo/fo-simple-page-master.h"

There is currently no policy on whether or not to include the subdirectory name when including headers in the same source code subdirectory.