Chad Perrin: SOB

24 March 2009

THAC0 was actually easy to use. No, really. I mean it.

Filed under: Geek,RPG — Tags: , — apotheon @ 03:27

This is part of my RPG series of entries here at SOB. See the inaugural entry in the series for more details.

Lost in the mists of time is the Second Edition of Advanced Dungeons and Dragons. AD&D 2E was the edition of the game that immediately (by which I mean “by about a decade or so”) preceded Third Edition Dungeons and Dragons. Back in those days, Armor Class was a number that was better when it was lower, ranging from 9 (if you had no armor) downward. One could have a negative AC, and that was a really good thing to have.

Spike Page at Ubiquitous Orcs asks Descending Armor Class ….Does ANYBODY still use it? The most important question asked there, I think, is not the query in the title. Rather, it’s an implied question. Spike says:

Now I normally consider myself to be just weird enough that I can actually find the bizarre logic behind just about anything, no matter how silly..but this one has me stumped. Perhaps descending AC is legacy rules from some old mothbally tactical historical game that TSR wrote back when TSR used to write such things..or maybe somebody wanted to make the math unnecessarily complicated so that non-nerds would shrink back in fear upon encountering such esoteric arithmatic.

But far be it from me to pontificate about which method of AC numbering is “right” and which isn’t. I certainly don’t mean to say anybody out there needs to change if they prefer the descending method…but I am genuninely curious as to why.

The implied question, then, is something like this:

What’s up with descending AC?

Most people (including Spike, probably), I think, would assume that the implied question is more like this:

Why do people use a difficult, math-heavy system of descending AC?

That version of the question assumes some things that simply aren’t true, though. The THAC0 system used in 2E isn’t any more difficult and math-heavy than the ascending system used in 3E. The problem isn’t THAC0; it’s the way THAC0 was explained in the books, and the fact that for some reason it seems like almost nobody ever noticed how easy it really is to use. I’m not really sure why it wasn’t figured out by more people, many of whom are quite intelligent (they’re gamers, after all).

What follows is a brief description of how you use the ascending AC from 3E.

player: I attack the orc. I rolled a 12. With my +5 Base Attack Bonus, that comes out to 17.

DM: (checks orc stats, finds that this orc has an AC of 16) That hits. Roll damage.

Next, I’ll provide an example of how most people used THAC0 in 2E, because this is how the 2E Player’s Handbook told them to do it.

player: I attack the orc. I rolled a 12. My THAC0 is 15.

DM: (checks orc stats, finds that this orc has an AC of 4, opens the PHB, finds that with a THAC0 of 15, it takes an 11 to hit the orc, forgets what the player said he rolled) I’m sorry — please remind me what you rolled.

player: 12.

DM: Oh. That hits. Roll damage.

Finally, I’ll provide an example of how THAC0 should be used in 2E.

player: I attack the orc. I rolled a 12. My THAC0 is 15.

DM: (checks orc stats, finds that this orc has an AC of 4, adds that to the roll of 12 for a total of 16, which is higher than the 15 THAC0) That hits. Roll damage.

See, the key is that descending AC isn’t a target number, nor is it a means of finding a target number on a chart, as most people thought. No, it’s a modifier to the roll. The target number is the THAC0.

It’s really that simple.

All that having been said, though, I still prefer an ascending AC. I just don’t see that it’s really all that big a deal.

The GURPS system, with its target numbers you have to roll under, is a real pain in the ass, though.


  1. Uh – in 2e, we made our players do the one bit of math, and if they roll 12 with a THACO of 15 they would say (15-12) “I hit AC3!” The DM would then say “hit!” It worked exactly like the current additive model, though admittedly doing a subtraction and possibly dealing with a negative number was “harder” or at least marginally slower.

    All additive is more elegant, but getting rid of THAC0 wasn’t one of the big selling points to us for going to 3e. (Now, going from the tables of 1e to the THAC0 of 2e – that was compelling.)

    Comment by mxyzplk — 24 March 2009 @ 04:53

  2. I’ve always gone with the first commenter’s system. It off-loads the GM a bit, letting them focus on the overall picture.

    Comment by mthomas768 — 24 March 2009 @ 05:17

  3. If you assume everybody knows the opponent’s AC, which you really should since what class of armor the opponent is wearing is visible to all, then the GM doesn’t have to do any lookups and the math for the players is the same as in ascending AC:

    GM: The Orcs are wearing Leather Armor and have Shields. Player: I roll a 12, plus 6 for their armor, my THAC0 is 15, so that hits.

    The whole problem stems from GMs treating AC as if it were a state secret instead of a class of equivalent armors.

    Comment by Joshua — 24 March 2009 @ 05:28

  4. @Joshua – except for magic on the armor, spell buffs, any special ability the players don’t know about, etc. There’s some reasons as a DM you might want to keep those secret, but even if you don’t believe in those being secret it’s poor GMing and leads to confusion. You certainly would need to explicitly tell the players the AC they’re dealing with, as their assumptions could only apply to humanoid opponents in seemingly-stock armor. Then, “the normal orcs are AC5, the sergeant is AC3, the witch doctor is AC2, and the wolves are AC7. OK, now keep that straight for a couple rounds.” Do you tell the players the monsters’ saves as well? No, it’s best for accuracy and efficiency if nothing else for the players to keep track of their copious stats and the DM to keep track of the opponents’, while offloading math to the players.

    Comment by mxyzplk — 24 March 2009 @ 07:30

  5. So you announce the AC. C’mon, it’s not like your players are incapable of noticing that “I hit AC 5” misses and “I hit AC 4” hits. Or, for that matter, “I got a total of 19” hits and “I got a total of 18” misses. The game assumes that combatants are decent judges of how hard it is to land a telling blow on a target…unless you’re going to have the GM make all the rolls in secret.

    Comment by Joshua — 24 March 2009 @ 07:40

  6. I’m still of the opinion that the easiest way to do it is to say “What’s your roll with all your modifiers? What’s your THAC0?” then add the opponent’s AC to it in your head (come on, it’s just a single modifier, it’s not that difficult) and compare to the THAC0 target. That is essentially exactly the same as the way the Base Attack Bonus and increasing AC system works, except that the attacker has the target number (not the defender), and the defender has the base attack modifier (not the attacker). Same system, different order. That’s it. Easy-peasey.

    Also . . . being told the ACs isn’t the same as figuring them out, especially since most of us aren’t counting cards at the Blackjack tables in Vegas.

    . . . but maybe that’s just me.

    Comment by apotheon — 24 March 2009 @ 11:17

  7. I always did it the way Mxyzplk describes, too, but I don’t recall ever having any particular trouble with it either way.

    I do prefer the 3e/4e ascending AC (and save bonuses/defenses), though. The consistent “bigger numbers are better” is nice.

    Comment by Scott — 25 March 2009 @ 01:37

  8. The nice thing about GURPS, though, is that it’s the same in every single case: having a higher stat in something means you are better at it, and the lower your score on the dice, the better. And of course, while THAC0 is a modifier to the target number upwards, which you can view as a bar to reach over, GURPS penalizes your skill for difficult tasks in exactly the same way, except that it decreases your skill level, which you then have to roll under. I don’t see the difference.

    Comment by sanbikinoraion — 25 March 2009 @ 03:32

  9. Scott:

    I do prefer the 3e/4e ascending AC (and save bonuses/defenses), though. The consistent “bigger numbers are better” is nice.

    I think the “bigger numbers are better” for AC provides more of a warm-and-fuzzy feeling when I look at my AC on my character sheet, so yeah — I tend to agree.


    GUPRS is certainly consistent in how its rules are used. It’s needlessly complex, though, and as I just hinted in my response to Scott using higher numbers to mean better numbers in all cases gives kind of a warm, fuzzy feeling that makes the game feel better somehow. I know that’s not just my taste talking, too; a lot of other people feel the same way.

    Besides, I’ve never been good at rolling low.

    Comment by apotheon — 25 March 2009 @ 06:35

  10. Hmm, I wouldn’t say that GURPS is needlessly complex – certainly not from the players’ perspective – I mean, there are hella lot of rules to cover pretty much every sort of combat or whatever you can think of, but it’s easy enough as a GM to simply say “you’re shooting lefthanded from 200 yards away in the dark, let’s call that -15 or so” so long as you have a reasonable feel for the numbers. You might argue that that is not actually playing the game – and maybe you’re right – but if we’re talking needless complexity and AD&D 2nd Ed, I think GURPS comes off fairly well in comparison :)

    (Remember the six pages giving ten different ways just to roll the dice to roll up your characters in the 2nd ed GM’s guide?)

    Comment by sanbikinoraion — 25 March 2009 @ 07:29

  11. Hmm, I wouldn’t say that GURPS is needlessly complex – certainly not from the players’ perspective

    I have to disagree. People complain about the exception-based system in D&D, but that’s nothing compared to character creation in GURPS, where almost every choice for every single aspect of character creation after buying attributes is its own exception, especially when making high point value characters. I’ve actually put off creating a character for a 500 point superhero GUPRS campaign for a little over a month now because of the work involved. It’s difficult during play, too — I specifically made my last 500 point superpowered character to make most of the combat capabilities of the character use nonvariable effects so that playing the character wouldn’t require nearly as much dice rolling or familiarity with nearly as many rules exceptions, thus greatly speeding up combat.

    Damage resolution is incredibly complex in GURPS, combining the most complex aspects of both the D&D Hit Point approach and the World of Darkness wounding level approach in such a way that new complexities are spawned from the void.

    if we’re talking needless complexity and AD&D 2nd Ed, I think GURPS comes off fairly well in comparison

    The needless complexity of AD&D 2E applied mostly in special cases that didn’t come up very often (I wasn’t making Petrify/Polymorph saves during every combat encounter), while the needless complexity of GURPS intrudes in pretty much everything one does, in my experience — starting with character creation.

    Remember the six pages giving ten different ways just to roll the dice to roll up your characters in the 2nd ed GM’s guide?

    Yeah, but you only use one of those attribute generation systems at a time, and the decision only has to be made once for the campaign (rather than once for every single character, for instance) — and the players usually never even see the choice being made.

    Comment by apotheon — 25 March 2009 @ 11:00

  12. Hmmm, I think I probably tend to play quite a rules-light version of GURPS so don’t tend to see the rules as all that fiddly – and since I usually play gunplay-oriented campaigns, the damage system is pretty simple because you don’t have to worry about different damage types. In fact, most of the time I just tend to say “roll some dice, yeah, that’s probably high enough” and only tot things up in the corner cases.

    … and when it comes to character creation, I don’t run high-end supers games, I generally run quite realistic, low-points games (I’ve gone as low as 25 points! That was fun :) ). So perhaps I’ve never really come up against the complicated advantage. Plus I quite enjoy making characters – I tend to get the players to give me an idea of what they want, and do the number crunching on their behalf.

    Anyway, you’re probably right; GURPS is quite complicated, I’ve just been playing it for so long that I tend to ignore all the hard parts :P

    Comment by sanbikinoraion — 26 March 2009 @ 03:05

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  14. I think the reason people hated THAC0 was that they thought they had to look in the books instead of just doing the maths. They thought that because Thieves and Wizards at 1st level had a THAC0 of 21, but with 20 for AC0. So you had to either remember THAC0 21 (but 20 for AC0) or look it up in a table. That persisted to 5th level for wizards, and a lot of people never really play much past that, so people probably thought that the table was dodgy somewhere else too.

    I liked the process of saying “I hit AC []”, and I also liked the negative ACs, they had a real feeling of importance, like AC 0 was some kind of barrier after which you got super cool. But in practice I prefer positive AC.

    Comment by faustusnotes — 1 April 2009 @ 01:25

  15. well, I’m just glad I never had to play with any of the retards that couldn’t do THAC0 in their heads after reading the explanation. It’s not confusing at all, people are just assholes. I totally lost interest in D&D when they discontinued the 2nd edition.. held on to the books for a few years, but realized it was a space-hogging effort in futility. Now I’m D&D free and couldn’t be happier.

    Comment by dave — 5 July 2010 @ 06:10

  16. Are you playing something other than D&D, or are you RPG-free in general?

    Comment by apotheon — 11 July 2010 @ 02:58

  17. I was a little skeptical when I saw the title of the article, but this is the first time I’ve read an article explaining THAC0 that actually makes sense. Most of them just recite the equations and expect you to follow them.

    Thanks for actually giving context on it.

    Comment by discord_inc — 22 September 2010 @ 03:40

  18. I’m glad you liked it. Welcome to SOB.

    By the way, appears to be broken. Is there a typo in that URL?

    Comment by apotheon — 22 September 2010 @ 04:49

  19. Yeah, it should have blogspot in there.

    Comment by discord_inc — 22 September 2010 @ 10:00

  20. Okay. I edited the URL on the previous comment.

    Comment by apotheon — 23 September 2010 @ 09:02

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