To Hit Descending AC

In the last entry, I discussed why I like descending AC and some of its advantages. I left the disadvantages for later, and so we have arrived.

My argument is simple: the bad parts of descending AC are the complicated ways used to determine how one beats the numbers, and ascending AC is therefore identified as the superior method only because it resolves the annoying math. However, I do not believe that one necessarily follows from the other; you can have descending AC without the complications. Before we get to that, let’s run through some of these methods and see where they cause friction, starting with the two methods that were used during AD&D’s run: To-Hit tables and THAC0. 

To-Hit tables were there from the start; the linked blogs concerning its genesis (found in the last post) are a recommended source to understand why the tables were built this way. The use is straightforward: find the monster’s AC and the PC’s level in the table headers, then cross-index to get the required value to hit. The 5th level Fighter is attacking an enemy with AC 5? Look at table I.B. on page 74 of the DMG and find that an 11 is needed to hit. The player rolls, adds any bonuses, and says, “I rolled a 12.” The DM says, “You hit.”

This is dead easy for the player, but compared to modern approaches it’s work for the DM. Instead of having to keep just a single number (AC) in their head and compare it against the player’s results, the DM has to reference a page of tables (or jot down the required values, for each level and class in the party, prior to play). Whichever way you look at it, having to reference a table each time a player smacks a goblin isn’t what I would call an elegant solution, though it was a very familiar state of play for early designers and DMs.

The state of play in the early '70s (Chainmail)

 

THAC0, or To-Hit Armor Class Zero, is a house-rule and cheat for resolving this lookup issue, made official in 2nd Edition. Since the original tables don’t change, their math can be worked backwards to give players a single number with which they can determine hits. Take the THAC0, subtract the AC of the monster, and ostensibly you’ve got your To-Hit number without any table lookups or fuss.

But of course, there is fuss. First, who does the math? If it’s the DM, then we’re just substituting one table for another, now with subtraction! If it’s the players, then are we giving out the monster’s AC? Online discussion suggests that it was common practice for players to subtract their attack roll from their THAC0 and announce which AC had been hit, though the original books don’t provide this particular formula. Even this is a kludge: you’re now asking players to roll high, add modifiers, and subtract two-digit numbers (which are easy to accidentally transpose) to reach a low target. Regardless of how much you can get used to it, this is why people talk about using THAC0 in the same way you’d describe using a slide rule.

Well, this is from 1e, but you get the idea.

 

Okay, those are the official ways to hit descending AC. What has the OSR developed? As noted in the prior post, most of them ditch it entirely, but for the few that keep the numbers heading toward zero, there’s little change. Games which hold to the original definition of OSR (i.e., cloning and cleaning up old-school D&D and AD&D) still stick to the tables or THAC0, while the “do whatever feels old-school, man” side of the OSR isn’t participating.

Frankly, that’s because there just aren’t many ways to move the numbers around, which is pretty well explained in this old Delta post. Here, the author proposes his more elegant Target20 system, which is the only alternate system to THAC0 that I’ve seen out there. With Target20, you add the attack roll, modifiers, level, and AC together in a single gumbo and try to hit 20. It’s far easier to wrap your head around than THAC0, but it’s still messy. Either you’re still sharing ACs with the players, or you’re tacking the AC onto the results that the players give you. It fails the elegance test when you’re having two people complete one equation.

Here's my take: I don’t see the point in trying to roll high to hit a low number. Instead, just roll low and provide negative modifiers to hit.

d20 - mods ≤ AC

This seems like an obvious solution to me, but it appears nowhere. The prevailing wisdom is that higher numbers are more fun and subtraction isn’t as easy as addition, but it’s not like there isn’t precedent for a race to the bottom rather than a climb to the top. Pre-Crisis D&D has plenty of checks that required you to roll low (without the need of a formula or cross referencing) accompanied by beneficial negative modifiers. In 2e this includes initiative, ability and proficiency checks, and all percentile rolls. Of course, AC also has negative bonuses.

If we were to build this low roll system based on 2e’s THAC0, the attack modifiers would be thus:

Group	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20
Warrior	-1	-2	-3	-4	-5	-6	-7	-8	-9	-10	-11	-12	-13	-14	-15	-16	-17	-18	-19	-20
Priest	-1	-1	-1	-3	-3	-3	-5	-5	-5	-7	-7	-7	-9	-9	-9	-11	-11	-11	-13	-13
Rogue	-1	-1	-2	-2	-3	-3	-4	-4	-5	-5	-6	-6	-7	-7	-8	-8	-9	-9	-10	-10
Wizard	-1	-1	-1	-2	-2	-2	-3	-3	-3	-4	-4	-4	-5	-5	-5	-6	-6	-6	-7	-7

Players record the negative modifier on their sheet upon level up. When attacking, the player can subtract their attack modifier from their d20 attack roll (along with any other situational or item modifiers) and state the results, and all the DM has to do is see if it’s equal to or lower than the monster’s AC. Done. It’s just the elegant ascending AC math flipped around. It’s even cleaner if you chop off everything past level 10; 1e had the right idea in expecting the characters to retire and become the Lord of Elfland or whatever.

This is just my take, but I think it’s worthy of play. It retains descending AC’s advantages, but provides the simplicity of ascending AC’s math. This solution’s absence from even one of the dizzying number of OSR games is mystifying, which I can only assume is due to a disdain for subtraction when calculating a good (dare I say, positive) result. Still, I think this method is a reasonable way to avoid throwing the AC baby out with the THAC0 bathwater.