It looks like this way of organising the source (src) tree started in 4.1BSD (1982?). Before that, all the commands lived under /usr/src/cmd/. See http://minnie

It looks like this way of organising the source (src) tree started in 4.1BSD (1982?). Before that, all the commands lived under /usr/src/cmd/. See http://minnie.tuhs.org/cgi-bin/utree.pl?file=4.1cBSD/usr/src http://minnie.tuhs.org/cgi-bin/utree.pl?file=4.1cBSD/usr/src and http://minnie.tuhs.org/cgi-bin/utree.pl?file=4BSD/usr/src http://minnie.tuhs.org/cgi-bin/utree.pl?file=4BSD/usr/src I guess it's there to flatten the source tree depth and probably had something to do with tiny disks of the time, shells without tab-complete or it was just a whim. In any case the convention stuck and it is now that way in all modern BSDs.