As a follow-up to this conversation about #ded #console #rulers, Thomas Dickey had this to say (in response to an email I'd sent him):

It's a visual reminder of how far left/right I've scrolled, in either the directory listing, or in the built-in pager. If you're using the pager, the scrolling applies to that.

Neither of those wraps lines, and when I
began that (in 1987), had no other programs to do that sort of thing.

If I'm scrolling left/right, I want to know where I am. If I scroll too far the right, and the screen's blank, that helps :-)

I see it's in the initial version of dedview.c, in April 1992, which precedes any of the followup versions of #dired. In turn, that says it was split from ded.c, so... in that file I see a ruler (in showFILES), which is from November 1987. The 'Xbase' variable corresponds to left/right scrolling - but that's the initial version of ded.c (I don't recall if the intermediate version that I wrote beginning around December/January 1986 did that, but have a hunch that this was something that I did for the improved version, with the ring of active directories, etc).

I had modified the initial version seen here:

https://groups.google.com/g/net.sources/c/_lGkTpsqyPE/m/SwhkKsBuayIJ

to move its divider line up and down in late 1983, and incorporated a similar divider into the versions that I wrote from scratch. I don't recall if I added column numbers to my changes in the original dired, but may have, since it seems "obvious".

Thanks for that, Thomas!!