Less Than THAC0

Thac0 was a part of AD&D?  Seriously?  Because I played AD&D (1st ed.) for a long time and wasn't aware of it.  Thanks to Cyclopeatron's research, I now know that Thac0 actually predates AD&D--or at least the DMG--but unless you were a computer nerd at UCLA in the late '70s, my guess is you probably didn't know why Thac0 was listed in statblocks for NPCs in the occasional module or what that unexplained column of numbers in Appendix E of the DMG labelled "To Hit A.C. 0" was all about.

Indeed, pointing to Appendix E as proof that Thac0 was a part of AD&D is like saying that "Less than Zero" was a Brad Pitt movie.  Sure he was in it--according to Imdb he was an uncredited partygoer--but it's only on because 2e adopted Thac0 full on--or so I gather--that this gains any significance at all.  If 2nd edition AD&D had gone with ascending ACs and to hit bonuses more akin to Castles & Crusades, et. al., Thac0 would be as familiar to us today as all those other party-going extras who did not become Brad Pitt. 

Thanks, but what's my Thac0?

Just as Andrew McCarthy, James Spader, and Robert Downey jr. were the headliners in the 1987 film of extreme LA youth decadence while Brad Pitt was an as yet unknown--except, perhaps, to a handful of "Growing Pains" devotees--the combat matrices on page 74-75 were the stars of the AD&D combat system.  When Aggro the Axe and Gutboy Barrelhouse squared off in the sample combat narrative on page 71 of the DMG, they used the combat matrices on the following pages to determine the success of their attack rolls, not some goofy acronym.  The continued absence of Thac0 rules in 1985's Unearthed Arcana is further indication that Thac0 was still not a significant part of AD&D, or had yet to be officially sanctioned by TSR anyway.

But obviously the Appendix E reference indicates that there was something going on with Thac0.  But did anyone outside of a posse of computer science majors at UCLA and a few Lake Geneva insiders actually know what Thac0 was supposed to be used for?  As a kid, I assumed that it was listed to give a measure of relative combat acumen--just as AC is a measure of defensive capacity--that could, in a pinch, be used to recreate the combat matrices.  Why anyone would bother doing such a  thing was beyond me since the matrices were readily available on every DM Screen that ever partitioned a gaming table.

But I could be totally wrong.  My sample size is not large; maybe loads of people were using Thac0 back in the day, despite TSR's refusal to endorse it.  Is there anyone out there that was into Thac0 before it went mainstream?  And what was the first TSR publication that actually explained what Thac0 was all about?