Useful for mounting the same directory in multiple places, particularly in chroot jails I hear.
# mount –bind olddir newdir
Mount a directory olddir as a mount point newdir (of course, mount point must exist!). Check using mount -l to see that it is in fact (rw,bind).
In /etc/fstab to mount on boot:
/path/to/olddir /path/to/newdir none rw,bind 0 0