Recently I came across some new schoolers at Penny Arcade comparing old school D&D to the version they play; they called it 4E or Double D or something. Hey, I'm just an unfrozen caveman, I don't understand these ascending armor classes or... well, that's pretty much all that I know about ND&D,* so I'll stop there. Seriously, it does not ruffle my feathers at all that there's a new version out there that's completely different from and incompatible with the game I played before the ice age claimed me back in the late 80s. Heck, I wanted to change that game too, can't blame TSR et. al. for doing the same thing and making money off of it. If the kids are havin' fun, then Game On!
*ND&D=New D&D, commonly denoted by a numerical "e rating," the higher the e-rating, the newer the D&D.
But there is one piece of information that I'd like to impart to the younger generations, and it's this: THAC0 and descending Armor Class are NOT synonymous.
Despite what this guy seems to think, if you mention THAC0 to stick-in-the-mud old schoolers like me who never graduated from the Gygax-authored tomes, there is no reason to expect them to know what you're talking about. Though the term Thac0 might have existed back in the day, it was not at all relevant to the game as it was played on the streets. The term came about, according to this research I poached from Philotomy, because of a curious column of data in the monster listings (Appendix E) of the AD&D Dungeon Masters Guide that was labeled "To Hit AC 0." However, there is no explanation of the significance of this this item and one is left to wonder why it was included with the other, more pertinent stats listed. Indeed, there's no reason I can see why they chose to spend 19 pages rehashing info that was already available in the Monster Manual when the only real value this table offers is looking up the XP value of that Shedu your players just blasted into a heap of fur and feathers (1,950 + 14/hp). No, when we old schoolers wanted to crush our foes we didn't call out our thac0s like a battle cry and have at it with our 20-siders; we turned to the combat tables on page 74-75 or eyeballed the DM's Screens that grew like stands of alder trees on gaming tables throughout the land. Then we walked 14 miles to school through a blizzard, uphill each way.
Seeing as Unearthed Arcana (1985) was the last new TSR-published D&D product I bought--though I played on for a couple more years in isolation from the machinations of Lake Geneva--I'm not entirely sure when thac0 grew to predominance. In fact, I wasn't aware that it had gained any traction until I read the Penny Arcade post referenced above. I'm guessing that it became prevalent in the 2nd Edition of The Game as a means to replace the combat tables as the go-to source for 20-sided slaughter in AD&D, a move in which I can definitely see some value. And presumably some game mechanics were changed somehow (I'm looking at you, repeating 20s) to make it more usable as a system. I'm not here to preach about the superiority of any one combat system over another--though, as much as I like the descending AC aesthetic, the ascending AC system sure makes life easier--but kids, get your facts straight.
So, yeah, I lived most of my life without giving Thac0 any thought whatsoever until these meddling kids started confusing descending AC values with this eldritch acronym. But now that my dander is up, why did they make it Thac0 and not Thac10? AC 10 is the Absolute of descending ACs; like 0 degrees Kelvin, you could only go up from there. And since no one needed a 20 to hit AC 10, you didn't need to worry where you stood on the ledge of repeating 20s. It does rhyme with the name of a major college sports conference of the western U.S., but that will likely be changing soon anyway. [Let's just say that Wazzoo won't be sandwiched between 2 filler helmets down at the bottom for much longer. Though the Cougs are certainly likely to stay at the bottom.]
Some have argued about the zero providing balance whereas using 10 as the base seems arbitrary and provides opportunity for unlimited growth and, therefore, AC inflation. I can sort of see what they're getting at; but unless you're in a game where there is a likelihood of finding suits of +12 plate mail or +24 rings of protection, there are probably de facto limits to AC inflation already in place.