On the Virtues of Descending AC

 I know, I know. Addition is easier than substraction, what kind of backwards grognard do you have to be to like descending AC in 2024? I get all that. But hear me out for a moment. My argument is that whilst ascending AC may be marginally easier to calculate to begin with, descending AC offers something different - A more intuitive appreciation of what the numbers mean and how they are bounded.

AD&D Armors

We'll start at the very beginning. Before that, even. An early draft for the first version of D&D:

Target20 was basically the original conception. Deduct AC from 20 and you have your attack target roll. Which is of course also how one converts descending AC to ascending. One wonders why they didn't just include this explanation to begin with, alongside a +to hit modifier, instead if messing with THAC0.

The math in the draft is a bit off, but it suggests another, even more intuitive, layer. If we stipulate that one must exceed the AC and not just meet it, it means that a regular character striking AC 2, plate mail and shield, has a 2-in-20 chance of hitting. Even though no published D&D has actually followed this math, I find this explanation a really good intuitive measure of armor.

Another thing to appreciate in this conception is that armor class is a "class". In the same way that travel fares have classes. Travelling first class is better than travelling third class. AC1, first class armor, is better than AC3, third class armor.

And with this in mind, we also get an intuitive appreciation of how far Armor Classes ought to span. 

AC 2 is achievable with Plate Mail and Shield. This "second class" armor was the lowest boundary in the original three booklets, as "Monsters and Treasures" have no critters with less than AC2 either, though magical armor could take it even lower. But essentially, "First class" armor was not even in scope for the original game. The intended range was 9 to 2. 8 Increments.

Even though the OD&D intended range was even more modest  than it needed to be, descending to AC us an intuitive understanding of the range of numbers we're operating with that is not evident in ascending AC. As we approach AC 0, we approach the bounds of reasonable armor classes.

In Greyhawk, for the first time, we see negative numbers. Dragons can go to -2. in B/X, a few critters (gold dragons, dragon turtles) go to -2. But very very few are at 0 or less. Once you get into negative numbers, you know you are truly dealing with something superhuman and/or supernatural. Essentially, game breaking armor classes.

I would argue AD&D unfortunately jumped the shark on this a bit. Extending unarmed to AC 10 may be a slightly more pleasing number, but the hit bonuses (notably fighter advancing it at every level, magic armor and weapons going all the way to +5) are not well aligned to the natural boundaries of descending AC. 

If your 14th level fighter has +20 to hit (+14 from level, +4 from magic weapon, +1 from STR. Not unreasonable assumptions at this level), then we start at negative numbers to make even mild challenges to hit. With percentile strength and a +5 weapon instead, it would cap at +22 to hit.

Conversely in B/X, a 14th fighter would be capped at a max of +15 to hit (+9 from level, +3 from STR 18, +3 max bonus from magical weapon). Still a bit high honestly, but this at least represents the maximum possible score in the game as written for B/X. at level 20 in AD&D, it caps at +26. That's a cap difference of 11. More than the full range of Armor Class as originally intended. An absolutely massive difference. 

Once you are in these numbers where negative AC becomes the baseline for any half decent foe, it ought to tell you that the numbers are getting too high. And if you are doing this with any kind of regularity, you really ought to be using ascending AC. 

Descending AC however, gives us a baseline, that tells us not to go there very often, that we are essentially stretching the numbers beyond their intended limits.  If you encounter negative AC, you know shit has hit the fan beyond peak human capacity. It tells us that this is an extraordinary encounter. 

And also informs GMs and game designers of the range of numbers the game wants to handle. I'd say to hit bonuses from level should also cap at +9 for sane numbers. similar to the OD&D start for unarmed AC. This is nearly what we get in B/X (caps at +10 for a 13th level fighter). And I'd cap AC at -2. That's 3 increments away from AC 1, our 'realistic' first class armor baseline. 3 increments is also the difference between AC 5 chain and AC 2 plate+shield, which is about as far as I can meaningfully stretch conceptualization of being that hard to hit, and keeps AC somewhat within similar bounds to the hit bonus.

The thing about this range is that the numbers are sane and somewhat bounded to a realistic range of 'hitability'. I don't think D&D was originally intended to emulate critters that are harder to hit than Erol Flynn dancing around in plate Mail and a shield (AC1), as evidenced by how AC2 is actually the lowest given for critters in the first three booklets. 

Most demons have negative AC in AD&D

Nor was it a game in need of emulating characters who can hit foes substantially harder to hit than that. But that is what happens in AD&D with its higher bonuses that goes off into mythic ranges and even more so in later editions, where the numbers truly begin to take off and we are now emulating true superheroes.

I don't expect D&D mechanics to be realistic, per se. But I do expect them to be somehow tied to some kind of real world measure, that gives you an idea of what it means in the world. And AD&D sort of lost sight of this measure, whilst later editions have simply jettisoned it altogether. 

I mean, what does -8 AC even mean? It's 10 increments removed from plate Mail and shield, whilst plate and shield is only 7 increments removed from being unarmed. How do you take the measure of that? And our 14th level fighter from above still hits that AC with greater ease than a 1st level fighter hits an unarmed foe. The numbers simply are no longer sane.

What it tells me is that classic D&D (including OD&D) remains the gold standard when it comes to the math of the game. Your mileage may vary.