Relocated Header Directories

[Dutch Ingraham <stoa@gmx.us>]

Hi everyone -

It seems Debian has moved some header directories, like /usr/include/bits (and
sys, and asm, etc.) from /usr/include/ to, e.g., /usr/include/i386-linux-gnu/bits/
(arch-specific).

My first question is:  Why?

My second question is: How does this work?  There are no symlinks, yet a file
like /usr/include/signal.h, has the standard "#include <bits/sigset.h>", yet
that path does not exist with the change noted above.  So how is this file
included?

Any enlightenment is appreciated.