It Made Sense at the Time: Descending AC

Iosue

Legend
When discussions about THAC0 come up, it's common for people to say something to the effect of "Why not just use Ascending AC? Why subtract? It's so weird and off-putting." And I don't really disagree, as far as THAC0 goes. It's not really that complex, and it's a perfectly cromulent system, but I can see why, especially from the perspective of the 30 and under crowd, it seems so rococo and counter-intuitive.

But to understand the Why of THAC0, of course you have to go back to the Why of Descending Armor class. And you have to understand that Everything was Tables. What I'd like to do here is explore how the road to THAC0, and then eventually Ascending Armor Class, was not a result of weird or even particularly idiosyncratic game design, but a progression of very logical steps, initially using assumptions (or One Big Assumption) a little different from what we have today.

D&D, as we all know, originally came out of wargaming. And certainly in the wargames of the '70s, but even into the '80s with such classics as Car Wars, Star Fleet Battles, and Battletech, tables were used to assist referees and/or players in determining the targets they wanted to hit with their rolls. The key understanding here is that there is generally one number to hit on the table, and naturally, it would fall within the range of the die or dice being used, a few situational bonuses notwithstanding. For example, here's the To-hit table for missile fire in Chainmail:
chainmail-fires-table-s.jpg

You cross reference the kind of missile weapon with the class of armor, at that range, and you have your target number. Chainmail is a 2d6 system, so you roll 2d6 and try to beat that number. Note the use of the term "class of armor." Ideally, for a fantasy battle game, you'd like a variety of armors. And then there will be monsters which won't have humanoid armor, per se, but will need to have target numbers. So the solution to this is have Armor Classes in lieu of a long list of individual artificial and natural armors (printing space is at a premium!). Now, certainly this could be anything. Armor Class A, B, and C. Armor Class First Grade, Second Grade, etc. Even Armor Class 1, 2, 3. The thing is, it doesn't matter what the Armor Class is called, because the target number is going to be on a table. Indeed, here, Armor Class 1 is in fact the weakest! (Also note here that 0 = 10, 1 = 11, and 2 = 12; printing space is at a premium!)

D&D was originally written to use Chainmail, but there is the alternative system. It's much simpler (no cross-referencing of weapon vs armor), and uses a d20 because that provides a very nice range of target numbers: the weakest armor class can be hit on a 10 (55% to hit), and the strongest armor class is hit on a 16 (25%). But here is where things get complicated. Because D&D puts a new spin on an idea from Chainmail: as your character gets more experience, their ability to hit improves. Okay, so you've got a variety of armors, of a successive difficulty to hit, so the target number has to go down as the character gains levels. And so it was:
men-attacking-b1-p19_orig.jpg


This design is not merely cromulent, it's both very standard for its time, and yet innovative in its introduction of character growth. The Referee checks the table, rolls, and immediately sees if its a hit or not, no math required. And if things stayed that way, there would be no problem. But, alas. First came players rolling their own dice. Then came STR bonuses (not in original D&D until Greyhawk!) added to magical weapon bonuses and negative ACs. Then some wise(acre) DMs figured out that if you just had the number to hit AC 0, and subtracted the enemy AC from that, you could figure out the target number, no table needed! But remembering everyone's to-hit for AC 0 is a pain in the butt, so you have the players remember it, and tell you what AC they hit with their roll (either by calculating THAC0 or looking at a table on their sheet). And the whole thing became a lot more math-y than just rolling, and cross referencing the result to a target number on a table.

The Big Assumption here is that: both the target number and the result would be one of the die faces. Even as the game expanded all way the to and beyond AC 0, it could be expected that the target number would be on the die, and so the result could be seen at a glance. Even those bonuses or penalties could be baked right into target number (either by character sheet tables or THAC0 calculations). If you time traveled to 1973, and told Gary Gygax, "Make the AC the target number!" He'd say, "But how do you handle improvement?" And when you say, "Attack bonus!" He'd say, "Well, that's eventually going to take the target numbers and the results off the range of the die. If you want numbers that big, you should just use a percentile system or something."

In essence, we could only arrive at Ascending AC through the route of tables -> off-the-cuff calculations -> inverting the calculations to reach "imaginary" target numbers. But each step made perfect sense, at its own particular time.
 

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rogueattorney

Adventurer
Right. At the beginning, armor class was just the heading at the top of a column in a non-linear chart. They could have named it anything. It was static in that it never changed unless your character changed his armor. There was no way to have a negative Armor Class in the original rules. Dexterity didn’t affect AC, nor did magical shields and armor, which affected the opponent’s hit roll and not AC.

As the rules got more complex, a lot of situational modifiers got added such that by the time 1e came out, your character’s AC would slide right and left on the chart based on circumstances, with the armor that the character wore only one factor in determining AC. Negative AC became possible, although I only ever commonly saw it in the 2e era video games like Baldur’s Gate, but maybe that was just me.

Gygax said he considered changing AC to ascending at the time of 1e’s publication, but decided for the sake of continuity to keep it as it was. And it is convenient for players of older versions that Chain and Shield was AC 4 in every (A)D&D product for 25 years no matter the edition.

THAC0 was an attempt to make things easier. It wasn’t fully useful in 1e and the D&D editions because of the non-linear nature of the charts. 2e “fixed” that by making the charts linear, effectively turning THAC0 from a limited tool to the rule. Again, probably a point where they should have changed AC to ascending, but again with the backwards compatibility issues.
 

GreyLord

Legend
Right. At the beginning, armor class was just the heading at the top of a column in a non-linear chart. They could have named it anything. It was static in that it never changed unless your character changed his armor. There was no way to have a negative Armor Class in the original rules. Dexterity didn’t affect AC, nor did magical shields and armor, which affected the opponent’s hit roll and not AC.

As the rules got more complex, a lot of situational modifiers got added such that by the time 1e came out, your character’s AC would slide right and left on the chart based on circumstances, with the armor that the character wore only one factor in determining AC. Negative AC became possible, although I only ever commonly saw it in the 2e era video games like Baldur’s Gate, but maybe that was just me.

Gygax said he considered changing AC to ascending at the time of 1e’s publication, but decided for the sake of continuity to keep it as it was. And it is convenient for players of older versions that Chain and Shield was AC 4 in every (A)D&D product for 25 years no matter the edition.

THAC0 was an attempt to make things easier. It wasn’t fully useful in 1e and the D&D editions because of the non-linear nature of the charts. 2e “fixed” that by making the charts linear, effectively turning THAC0 from a limited tool to the rule. Again, probably a point where they should have changed AC to ascending, but again with the backwards compatibility issues.

Interestingly enough, T.H.A.C.0 first appears in AD&D. You can even find the THACO listed in the DMG. THAC0 did NOT mysteriously appear in 2e. It appeared in 1e. It appeared pretty early on in 1e's life as well.

What 2e did was mathematically codify it for classes far more than 1e did and make it more compatible with higher levels and rolls above 20 (which I think they did away with, at least in theory, in 2e. I don't think higher levels with easy hits and 20+ is even listed in 2e).
 

I don't know about Chainmail, but I know there are wargames where you roll what amounts to a saving throw against your own defensive rating to avoid taking hits - e.g. naval combat defences in World in Flames, for instance; in that game, as with old-school AC, a lower defence rating is better. Unlike D&D, it's because you want to roll over it in order to make your save. But the idea that lower defensive ratings are better isn't just because of the connotation of class/rank ("first-class", etc.); in many wargames it may well have a direct mechanical effect.
 

James Gasik

We don't talk about Pun-Pun
Supporter
The Big Assumption here is that: both the target number and the result would be one of the die faces. Even as the game expanded all way the to and beyond AC 0, it could be expected that the target number would be on the die, and so the result could be seen at a glance. Even those bonuses or penalties could be baked right into target number (either by character sheet tables or THAC0 calculations). If you time traveled to 1973, and told Gary Gygax, "Make the AC the target number!" He'd say, "But how do you handle improvement?" And when you say, "Attack bonus!" He'd say, "Well, that's eventually going to take the target numbers and the results off the range of the die. If you want numbers that big, you should just use a percentile system or something."

In essence, we could only arrive at Ascending AC through the route of tables -> off-the-cuff calculations -> inverting the calculations to reach "imaginary" target numbers. But each step made perfect sense, at its own particular time.
Of course, then in 2e, they decided taking the numbers off the die was just fine with the Great Wyrm Red and Gold Dragons, lol.
 

Enrahim2

Adventurer
Indeed, here, Armor Class 1 is in fact the weakest!
This again hints to me that Gygax really tried to make the alternative rules in the original D&D box inconvenient on purpose in order to drive chainmail sales. After all the main system used ascending AC, and why would he consider ascending AC for AD&D first edition, if he wasn't aware of benefits to it? (Using the less available d20 rather than 2d6 is also a hint Gygax really didn't want people to use the alternative system in the D&D beyond as a quickstart demo for the real game using chainmail :p )
 

I rather liked descending AC. It does take getting used to for sure. Overall I found the bonuses and rolls felt tighter in both versions of AD&D than they did in 3E (some of those rolls in the 3E era got crazy). Liked roll under NWPs as well.

With THAC0 in particular, I find that can be tricky for new players at first but once you explain the way to calculate it, it isn't too much trouble (and players can make cheat sheets for their characters to make it easier).
 

James Gasik

We don't talk about Pun-Pun
Supporter
I rather liked descending AC. It does take getting used to for sure. Overall I found the bonuses and rolls felt tighter in both versions of AD&D than they did in 3E (some of those rolls in the 3E era got crazy). Liked roll under NWPs as well.

With THAC0 in particular, I find that can be tricky for new players at first but once you explain the way to calculate it, it isn't too much trouble (and players can make cheat sheets for their characters to make it easier).
Yeah, it's not that Thac0 is impossible, but some people find ascending easier to at least some degree. Mostly it's that, neither way is superior to the other, so if someone has learned to do it one way (say, 5e gamers I tried to introduce 2e to, lol*), they're not going to see any advantage to doing it the other way.

*(Or my 2e DM who grumbles mightily whenever we ask him to roll a d20 in 5e.)
 


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