@nixCraft Basically correct but mistaken in one or two minor details.

The origin of /usr/bin and /usr/lib is as described, but original ken/dmr Unix had no sbin. sbin these days is taken to mean programs meant only for sysadmins; in the early such programs were in /etc or just /bin.

As I vaguely remember--and I may be mistaken myself here--the s in /sbin originally meant statically-linked, programs like mount that were need to make /usr and hence shared libraries available. (What about /bin/sh? I don't remember.) In that meaning there was no /usr/sbin; it made no sense.

As an old-timer who is always looking to make things smaller and simpler, were it up to me (and it surely is not), I'd abolish /usr except as a place for users' home directories. And I'd abolish /etc as well: executables have a good home now in /sbin, config/data files should move to /lib, which was originally a general library of files need by programs in /bin, not just subroutine libraries.