(6.8.2) The method for locating includable source files:

A file whose name is delimited by < > is searched for first in the directories named by the -I option, and then in the standard directory. The standard directory is /usr/include, unless the -YI option is used to specify a different default location.

A file whose name is delimited by quotes is searched for first in the directory of the source file that contains the #include, then in directories named by the -I option, and last in the standard directory.

If a file name enclosed in < > or double quotes begins with a / character, the file name is interpreted as a path name beginning in the root directory. The search for this file begins in the root directory only.