In a private conversation some years ago, Tim BL told me that he used to use Apollo workstations back in the 1980s and that he really liked Apollo Domain/OS, so he took the // from the Domain distributed filesystem, which used // as a way of addressing possibly remote files, i.e., //hostname/path/to/file . I suspect the \\ in Microsoft UNC pathnames is also derived from the same, probably due to Paul Leach's influence there as he was also from Apollo.