AC: Up or Down, Who Cares?

There’s an old argument in the OSR about ascending versus descending Armor Class, and I’ve never quite understood why it sometimes gets treated as a mechanical argument instead of a taste argument. Of course, I’m a few decades late in weighing in on this one. Since I plan to run B/X D&D on our podcast in the future but use ascending AC (heresy!), this seems a good time to speak up. I grew up with descending AC, but I prefer ascending AC, simply because it makes more sense to me. I don't have to stop and do math in my head or look something up on a matrix--I roll, add my bonus, and see if I hit. It’s just my personal preference. I think it’s wrong to suggest, as grumpy old gamers sometimes do, that descending AC is superior in terms of game mechanics. It’s not. Descending or ascending, it all shakes out the same. 

In old-school D&D, AC is descending: unarmored is AC 9, leather is AC 7, chain is AC 5, plate is AC 3, and so on. Lower is better. In ascending AC, those numbers just go the other way: unarmored might be AC 10, leather 12, chain 14, plate 16. Higher is better. I have absolutely no problem with somebody preferring descending AC. If you grew up with it, like I did, it feels like D&D. If you like the old terminology, the old character sheets, or compatibility with old adventure modules and bestiaries, that's perfectly fine. But I don't think there's any inherent mechanical advantage to it. You're basically taking the same numbers and turning them upside down.

And yes, before somebody points it out, I know there's an apparent contradiction in Old School Essentials, one of the most popular clones of old D&D. It defaults to descending AC, but offers an ascending option. OSE's old-school attack matrix, its THAC0 formula, and its ascending-AC system don't produce exactly the same probabilities at the extreme ends of the matrix, and at first, I thought this meant I was wrong. But when I do the math (not always easy for me, I admit) that's not evidence that descending AC is somehow mathematically better. It's a quirk of the old attack matrix. Because the matrix has to deal with the fact that you're rolling a d20: when to hit would require something higher than a 20, the table just says 20; and at the other end, it bottoms out at 2 because a natural 1 always misses. So you can get a tiny difference between what the old matrix says and what the straight formula would say. That's interesting, and worth pointing out, but it has nothing to do with whether AC goes up or down. It's a quirk of the table. It seems to me you could build an ascending system with exactly the same quirk.

And that's really where I think the argument gets muddy--confusing "I like it better" with "it is objectively better." If somebody tells me, "I prefer descending AC because it feels better," I'm not going to argue with them. If they tell me, "I've been playing this way for thirty years and I don't see any point in changing," fine. If seeing AC 2 on the character sheet makes the game feel like D&D in a way that AC 18 doesn't, I understand that. But those are arguments about taste, aesthetics, and nostalgia. They're not mechanical arguments. Descending AC wasn't invented because some clever primordial nerd discovered that making the numbers go downward produced better combat. It was simply the way the original game represented the numbers, and some folks have been carrying that around ever since. Sometimes the OSR has a tendency to assume that because a rule is old, there must be some profound design wisdom behind it.  But sometimes an old rule is just an old rule.

To be clear, I'm not arguing that ascending AC is better. That's not even remotely my point. If you like descending AC, use descending AC. I like plenty of old-school "legacy affectations" myself, and I certainly don't think every modern design is an improvement. All I'm saying is that there's nothing inherently superior about AC 2 compared to AC 18. Armor Class is just a number representing how difficult something is to hit. We can make that number go up or make it go down, and the game works just fine either way. Like descending AC all you want. Just don't confuse liking it with having discovered a mechanical advantage.

And by the way, if you want to convert traditional descending AC to ascending AC, just subtract the old AC from 19. AC 5 becomes AC 14, AC 3 becomes AC 16, AC 0 becomes AC 19. That's basically the whole trick.