This is part of my RPG series of entries here at SOB. See the inaugural entry in the series for more details.

Lost in the mists of time is the Second Edition of Advanced Dungeons and Dragons. AD&D 2E was the edition of the game that immediately (by which I mean “by about a decade or so”) preceded Third Edition Dungeons and Dragons. Back in those days, Armor Class was a number that was better when it was lower, ranging from 9 (if you had no armor) downward. One could have a negative AC, and that was a really good thing to have.

Spike Page at Ubiquitous Orcs asks Descending Armor Class ….Does ANYBODY still use it? The most important question asked there, I think, is not the query in the title. Rather, it’s an implied question. Spike says:

Now I normally consider myself to be just weird enough that I can actually find the bizarre logic behind just about anything, no matter how silly..but this one has me stumped. Perhaps descending AC is legacy rules from some old mothbally tactical historical game that TSR wrote back when TSR used to write such things..or maybe somebody wanted to make the math unnecessarily complicated so that non-nerds would shrink back in fear upon encountering such esoteric arithmatic. But far be it from me to pontificate about which method of AC numbering is “right” and which isn’t. I certainly don’t mean to say anybody out there needs to change if they prefer the descending method…but I am genuninely curious as to why.

The implied question, then, is something like this:

What’s up with descending AC?

Most people (including Spike, probably), I think, would assume that the implied question is more like this:

Why do people use a difficult, math-heavy system of descending AC?

That version of the question assumes some things that simply aren’t true, though. The THAC0 system used in 2E isn’t any more difficult and math-heavy than the ascending system used in 3E. The problem isn’t THAC0; it’s the way THAC0 was explained in the books, and the fact that for some reason it seems like almost nobody ever noticed how easy it really is to use. I’m not really sure why it wasn’t figured out by more people, many of whom are quite intelligent (they’re gamers, after all).

What follows is a brief description of how you use the ascending AC from 3E.

player: I attack the orc. I rolled a 12. With my +5 Base Attack Bonus, that comes out to 17. DM: (checks orc stats, finds that this orc has an AC of 16) That hits. Roll damage.

Next, I’ll provide an example of how most people used THAC0 in 2E, because this is how the 2E Player’s Handbook told them to do it.

player: I attack the orc. I rolled a 12. My THAC0 is 15. DM: (checks orc stats, finds that this orc has an AC of 4, opens the PHB, finds that with a THAC0 of 15, it takes an 11 to hit the orc, forgets what the player said he rolled) I’m sorry — please remind me what you rolled. player: 12. DM: Oh. That hits. Roll damage.

Finally, I’ll provide an example of how THAC0 should be used in 2E.

player: I attack the orc. I rolled a 12. My THAC0 is 15. DM: (checks orc stats, finds that this orc has an AC of 4, adds that to the roll of 12 for a total of 16, which is higher than the 15 THAC0) That hits. Roll damage.

See, the key is that descending AC isn’t a target number, nor is it a means of finding a target number on a chart, as most people thought. No, it’s a modifier to the roll. The target number is the THAC0.

It’s really that simple.

All that having been said, though, I still prefer an ascending AC. I just don’t see that it’s really all that big a deal.

The GURPS system, with its target numbers you have to roll under, is a real pain in the ass, though.