In the old days, back when disks were smaller, Unix and Linux systems tended to have the “/” (root) and “/usr” directories on different volumes. In fact, I can remember that the DEC/Compaq Tru64 UNIX installer insisted on arranging things this way, even though the disks were quite big enough by then to put the whole OS on one volume. (I just moved /usr into the root volume afterwards and removed the redundant mount point.) One consequence of this was that you had to be careful where you put core parts of the system: things had to be designed so that you would still have some useable functionality if you only had / mounted and not /usr (for recovery purposes). Thus, binaries in /bin and /sbin could depend on shareable libraries in /lib, but not /usr/lib, while of course binaries in /usr/bin and /usr/sbin could freely depend on both. These days, nobody cares about such a possibility any more. Other distros (notably Red Hat) abandoned the root-usr distinction a long time ago, and merged things so /bin was just a symlink to /usr/bin, and /lib to /usr/lib, anyway. You can still put /usr on a separate volume, but your system will no longer work if it cannot be mounted. Now Debian is following suit. <http://www.theregister.co.uk/2016/11/24/debian_testing_merged_codeusrcode/>