Descending Armor Class

I hail from the era of the post-3e consensus, where it is a given that descending armor class is unintuitive and dumb, while ascending armor class is simple and pristine. The orthodoxy on this is rather ubiquitous. I’ve been hard pressed to find any modern TTRPG that embraces the concept, and even the OSR gang tends to toss the idea overboard when designing their fantasy heartbreakers.

The origins of descending armor class are murky, though it’s been well explored in blogs that actually know what they’re talking about. The apocrypha suggests that it was built from Arneson’s experience with naval wargaming, where a ship with 1st class armor would be harder to sink than 2nd class armor. While that is likely the origin of why we call it “Armor Class,” the choice to descend appears to be based more on Gary’s opinion on the elegance of the math involved.

Elegance

Putting aside the math for a later date, one may ask if there are any advantages to descending armor class, at least in the context of designing new or derivative rules for TTRPG combat.

I can see three.

First, it’s compatible with TSR-era modules. When 25 years of D&D material uses descending AC, using that expected number keeps DMs from having to do quick conversion or scribble in the margins. There’s value in being able to run the old stuff right off the page without much adjustment, even if the math gets a bit off between the early editions.

Second, it evokes an old-school feel. This is admittedly trivial, but it’s not to be discounted. Much of a game’s tone and style is brought forth from the decisions a designer makes in crafting its ruleset. There’s nothing that says ‘old game’ like holding onto an antique like descending AC. It’s like putting a spittoon in your bar or using neon pink in your cyberpunk art; the aesthetic is in service to the ideal.

Or like putting this style of art in your AD&D clone.

Third, a bounded range that terminates at zero provides a floor. AC 0 stands as a solid barrier between a character that is just well protected or quick and one that is armored by magic or by other extraordinary means. When crossing that threshold of zero into the negative, PCs find themselves fighting something otherworldly. Those monsters are a mathematical indication that you have entered the Upside Down. This is compared to something like 5e, where the barrier of 20 is rather soft; the difference between an enemy with AC 19 and AC 21 is just a small bump in difficulty, but the difference between AC 1 and AC -1 is a line between the mundane and the mythical. This Mythlands post does a great job of digging into the idea. 

These advantages suggest to me that descending AC should hold at least some esteem in modern old-school creations, but it’s generally rejected unless the work is trying to ensure compatibility. The reason for this has nothing to do with AC moving south, but instead is due to the mechanism for how you hit it. Tables and wonky formulas are what get brought up when descending AC is derided. We'll look at that problem next time.