George Michaelson recounts some interesting history from the DEC world.
https://www.tuhs.org/pipermail/tuhs/2025-April/031770.html
From the other end of the spectrum, it is interesting to observe the reasons that #POSIX initially did not specify what happened when one did ".." from the root directory. It wasn't just because of the Andrew File System. POSIX defined an API, and theoretically one that could be provided by something that was not Unix, that did not use Unix filesystems.
Early #FAT did not have a ",," entry in the root, or even subdirectories at all in very early versions of MS/PC-DOS. ".." was wasteful of limited root directory space until FAT32 came along and root directory handling stopped having to always be a special case anyway.
#HPFS did not have "." or ".." entries on-disc at all. They were synthesized by the filesystem driver from a special start record. But the root directory had one of those.
Of course, NTFS just brings us back to the #DEC world. (-:
#FilesystemFormats #OS2