Ever wonder why UNIX/Linux has so many random directories for storing binaries, libraries, and so on (e.g. /bin, /sbin, /usr/bin, /usr/sbin, /usr/local/bin), and why user directories are actually under /home rather than /usr? Well, wonder no more, thanks to Rob Landley and a nice comment thread at Hacker News. Rob starts out:

You know how Ken Thompson and Dennis Ritchie created Unix on a PDP-7 in 1969? Well around 1971 they upgraded to a PDP-11 with a pair of RK05 disk packs (1.5 megabytes each) for storage.

When the operating system grew too big to fit on the first RK05 disk pack (their root filesystem) they let it leak into the second one, which is where all the user home directories lived (which is why the mount was called /usr). They replicated all the OS directories under there (/bin, /sbin, /lib, /tmp…) and wrote files to those new directories because their original disk was out of space. When they got a third disk, they mounted it on /home and relocated all the user directories to there so the OS could consume all the space on both disks and grow to THREE WHOLE MEGABYTES (ooooh!).