What are GCC's "Standard system directories" on GNU/Linux?

Hello, comp.lang.c.

In the GCC manual, section 3.16 "Options for Directory Search" partially
describes where, how, and in what order GCC finds #include files when
compiling.

It's the "partially" bit which is getting on my nerves.  The manual
section contains a priority list for finding #include files, but the
fifth item just vaguely states:

    5. Standard system directories are scanned.

.  Which directories are these?  Where is this documented?

I'm not stupid, I know vaguely what the purpose of these directories is.
But I need to know precisely which directories they are.

GCC was built mainly for the GNU project.  Why must the manual be so
vague on this point, even for GNU/Linux?

--
Alan Mackenzie (Nuremberg, Germany).