#### Sacrosanct

##### Legend
I'll repeat what I said earlier, because apparently it's been ignored judging by the many posts since. It's not that subtracting is harder than adding. It's that the difference is incredibly minor that it shouldn't have any measurable effect on length or difficulty of resolving combat. You're not being asked to factor polynomials here folks. You're being asked to subtract from no more than 20. That's first grade stuff. If you have a difficult time calculating "I rolled a 14, my THAC0 is 18, so I hit AC 4", then I suspect you're having difficulty with every mechanical aspect of the game. I mean, you're constantly doing harder math every time you gain or spend coins (gp, sp, etc).

#### Tony Vargas

##### Legend
I'll repeat what I said earlier, because apparently it's been ignored judging by the many posts since. It's not that subtracting is harder than adding. It's that the difference is incredibly minor that it shouldn't have any measurable effect on length or difficulty of resolving combat. You're not being asked to factor polynomials here folks. You're being asked to subtract from no more than 20. That's first grade stuff. If you have a difficult time calculating "I rolled a 14, my THAC0 is 18, so I hit AC 4",
Yeah, but, why should I bother with all this new-fangled Thack-hoe stuff, when I have the attack matrices right here on my handy-dandy DM screen?

Standing by un-measurable assertions is easy. It still looks like standing by your 1974 Ford Pinto* as a great car.
*Which had a reputation for blowing up if it got rear-ended
I think his assertion in this analogy would be that his cherry Ford Pinto is not nearly as dangerous a car as it was made out to be. And he'd be right - very few Pintos actually exploded.

If you're gonna use an analogy, use a correct one. D&D was one of the best RPGs back then, so you have to use a good car...
True, in 1974, it was the only RPG. If D&D was a Ford by analogy, it'd be a Model T, for certain. And the Model T remains one of the best-selling cars in history.

So I totally get standing by your Model T. Or Model A. You own a piece of history.

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#### Charlaquin

##### Goblin Queen
So, to actually add something to this discussion, while addition may be easier for more people than subtraction, and higher AC being better may be more intuitive, THAC0 does have one major advantage over BAB: With THAC0, you do the math to figure out what your target number is before you roll, so you immediately know if you hit or miss as soon as the die stops rolling. This as opposed to BAB where you roll first, add your modifiers, and then compare the total to the target number.

Both probably take about the same amount of time and effort, but by front-loading the math, THAC0 makes resolving an attack a miniature dramatic tension arc. You start from the lowest point of tension as you figure out the number you need on the die, then get a little rise of excitement as you roll the die around in your hand, the anticipation as you let it roll onto the table, coming to a climax just before the die stops rolling, and then immediate release as you see the result and know instantly if it is high enough or not. With BAB, that arc gets disrupted by having to add your modifier to the roll result in between the climax and the denouement, which while brief, does make the overall experience less viscerally satisfying. It’s a small thing, but I suspect it’s a big part of why the folks who loved THAC0 miss it, even if not consciously.

What I would like to see is a system that combined the strengths of both approaches. I already tell my players the DC of a check when I call for one to be made, perhaps it would be best to go a step further. Instead of a positive modifier to the roll, give the players a negative modifier to the target number. Sure, then you’re back to subtraction instead of addition, but I suspect it would still be more intuitive than THAC0 due to higher AC still being better, and “subtract your modifier from the target’s AC” being a cleaner way of presenting the same math than “subtract the target’s AC from a number derived from your class and level.”

For me THACO always made me pause to figure out the math every single time especially with switching gears for where the magic/strength/specialization bonus goes into the equation.

So THACO minus AC compare to roll as modified by bonuses.

"I've got a THACO of 17, I have a +2 to hit, I rolled an 11 so I hit an AC of . . . 4?" It was a little speed bump every time.

And I'd have players asking does a 12 hit? So I'd ask for their THACO and their bonuses then plug in the roll and the monster's AC to figure it out and. It was annoying.

"I've got a BAB of +3 with a +2 bonus and I rolled an 11 so I hit an AC of 16" Was quicker and less shifting of mental gears so in play it went faster and kept me more in the narrative flow of a combat and less in calculating the numbers mechanics.

I was excited when I saw it in 2e Dragon Fist and was glad it was in 3e and everything since.

#### lowkey13

##### I'm sorry, Dave. I'm afraid I can't do that.
Both probably take about the same amount of time and effort, but by front-loading the math, THAC0 makes resolving an attack a miniature dramatic tension arc. You start from the lowest point of tension as you figure out the number you need on the die, then get a little rise of excitement as you roll the die around in your hand, the anticipation as you let it roll onto the table, coming to a climax just before the die stops rolling, and then immediate release as you see the result and know instantly if it is high enough or not. With BAB, that arc gets disrupted by having to add your modifier to the roll result in between the climax and the denouement, which while brief, does make the overall experience less viscerally satisfying. It’s a small thing, but I suspect it’s a big part of why the folks who loved THAC0 miss it, even if not consciously.
That's right.

THAC0 is Christmas morning.

BAB is opening your presents, and then hearing, "Yeah, sorry, I forgot to get the batteries. And nothing is open. You don't mind waiting, do you?"

#### billd91

##### Hobbit on Quest
So, to actually add something to this discussion, while addition may be easier for more people than subtraction, and higher AC being better may be more intuitive, THAC0 does have one major advantage over BAB: With THAC0, you do the math to figure out what your target number is before you roll, so you immediately know if you hit or miss as soon as the die stops rolling. This as opposed to BAB where you roll first, add your modifiers, and then compare the total to the target number.
Why would you know your target number before you roll with THAC0? Is the DM telling you your opponent's AC?

#### Charlaquin

##### Goblin Queen
Why would you know your target number before you roll with THAC0? Is the DM telling you your opponent's AC?
Did you do it differently? I guess I never considered the possibility that the DM might just ask you to roll and tell him the number you got and your THAC0, do the math himself, and tell you if you hit or not. That sounds awful, it’s no wonder it developed such a bad reputation if folks were doing it that way.

#### alienux

##### Explorer
I never thought THAC0 was bad, or realized anyone else did, until I started hearing people complain about it on the Internet years later.

##### Archivist
There's more historical revisionism going on here than a typical grade school U.S. History class.

THAC0 was awful. Negative AC was awful and un-intuitive. We only put up with it because we didn't know any better.

Now we do.

#### Fenris-77

##### Explorer
I never had any problems with THAC0 nor did any of my regular groups, at least to the best of my recollection. I always did the initial calculations for my characters weapons ahead of time and noted it on my character sheet. What I did really like about the THAC0 system was the immense sense of satisfaction when you finally got a fighter down to a negative AC. Then you were in business. I guess getting over 20 might feel like that to someone who never played with THAC0? I don't know, the current system, as much as it makes sense, and is elegant design-wise comparatively, still doesn't "feel like D&D" to me.

#### Bacon Bits

I have not read the rest of the thread; however, I have opinions.

I feel like THAC0 gets a bad rap. It's the precursor to 3E's Base Attack Bonus, and came after 1E's attack matrices. It's actually super simple.

The important thing to remember is that it's the exact same math, but in reverse.

In 3E-5E, you roll d20 and add a bonus in order to beat the target's AC.

In 1E-2E, you roll d20 in order to beat THAC0 minus AC.​

If your THAC0 is 15, and your target's AC is 2, you need to roll 13. It's that simple! (Remember, lower AC was better).

I feel sorry for poor old THAC0. It has a bad reputation for being complex, when it's exactly as complex as the current method!
No, it's not just as complex as the current method. It's only the same complexity when you already know the target's AC. Since AC varies between targets and is typically a secret number until the players manage to work it out, THAC0 is not less complex. Players can't do die roll + AC >= THAC0. Players have to do Die roll - THAC0 = AC hit. Except that's wrong. They actually have to do THAC0 - Die roll = AC hit. Did you notice the error while reading, or did you have to stop to think about it? Is it obvious which is correct? If your THAC0 is 15 and your modified roll is 13, is it easy to intuit while rolling the dice that you hit AC 2 and not AC -2? Not really. It's hard to look at the dice and say, "Okay, that's not possible you must've done the maths wrong because that result is way off." You can't do that because 90% of the time your results are between -5 and 5. All your numbers look the same. And now you add in a modifer, and it gets really complicated. See, it's really easy to confuse THAC0 - Die roll - 1 = AC hit and THAC0 - (Die roll - 1) = AC hit. The room for math errors in how people actually use THAC0 is significant.

The real problem with THAC0 is that the "easy" mode formula of die roll + AC >= THAC0 conflicts with player nature. When players roll an attack die, every player -- and I do mean every player -- wants to know "What AC did I hit?" That's the question they always pose. It's one of the most common questions asked at the AD&D table. Players can't measure in their performance *against the other PCs and NPCs* without doing that step. In order to understand how well their character is doing -- how powerful they are -- knowing the target number they reached is vital. THAC0, in it's "easy" mode, seeks to obscure that information from the player by design. Gygax assumed the player would never know their THAC0. That's why the charts were in the DMG. Gygax assumed only the DM would determine the outcome of an attack because the DM would be in total control of everything. In reality, that's too much burden on the DM to look up tables all the time. So everybody made the players do it. Except the players want to know how well they're doing. Unless they roll high or low, they won't know. And then they'll only know they rolled high or low.

The lesson of THAC0 and descending armor class is clear: You cannot design an action resolution system around how you want people to play. You need to design them around what makes game play operate swiftly, intuitively for the players and the DM, it must be easy to evaluate by the players and DM so they can understand the consequences of modifiers, and it must help all players and DMs at the table understand what's going on and maintain the suspension of disbelief. It must be fast to resolve, it must not be prone to simple math errors, and it must not occupy more game time than it absolutely must. THAC0 and descending armor class essentially do none of that.

#### Charlaquin

##### Goblin Queen
There's more historical revisionism going on here than a typical grade school U.S. History class.

THAC0 was awful. Negative AC was awful and un-intuitive. We only put up with it because we didn't know any better.

Now we do.
Maybe for your group. For as long as I’ve been discussing games on the internet there have been people who claimed to have genuinely preferred THAC0. Personally, I didn’t start playing until 3.5, my analysis is based purely on my understanding of game design and psychology.

#### Charlaquin

##### Goblin Queen
Why would you know your target number before you roll with THAC0? Is the DM telling you your opponent's AC?
No, it's not just as complex as the current method. It's only the same complexity when you already know the target's AC. Since AC varies between targets and is typically a secret number until the players manage to work it out, THAC0 is not less complex.
You know, I think I may have discovered the key factor in whether or not someone liked THAC0.

#### Slit518

##### Explorer
I feel like THAC0 gets a bad rap. It's the precursor to 3E's Base Attack Bonus, and came after 1E's attack matrices. It's actually super simple.

The important thing to remember is that it's the exact same math, but in reverse.

In 3E-5E, you roll d20 and add a bonus in order to beat the target's AC.

In 1E-2E, you roll d20 in order to beat THAC0 minus AC.​

If your THAC0 is 15, and your target's AC is 2, you need to roll 13. It's that simple! (Remember, lower AC was better).

I feel sorry for poor old THAC0. It has a bad reputation for being complex, when it's exactly as complex as the current method!
I started with 2nd edition in 1999 when I was in high school.

Never had a problem with it, and enjoyed it.

Getting Armor Class to the negatives was a fun challenge.

I too miss Thac0.

For those of you who don't know, in case it wasn't mentioned - Thac0 stands for, "To Hit Armor Class 0."
So, if my character's Armor Class was 0, your Thac0 would tell you what number on the d20 you would need to roll (or higher) in order to hit me.

Thac0 - pronounced: th.aaa.co

THAC0, in it's "easy" mode, seeks to obscure that information from the player by design. Gygax assumed the player would never know their THAC0. That's why the charts were in the DMG. Gygax assumed only the DM would determine the outcome of an attack because the DM would be in total control of everything.
I think this is a good point. I remember trying to do that as a DM, to have the player roll and know that rolling high is better and that they have some bonuses and that some classes are better than others, but I as the DM would handle the specifics and they could focus on the experience and not worry about the mechanical numbers. In essence I would be most of the computer handling the mechanics behind the scenes and they could have more of a narrative experience.

But that is a big pain to handle as a DM.

Also many people are both players and DMs so they can legitimately see the stuff in the DMG so the pure player experience is an obvious myth, many players know the mechanics.

Then there were 1e character sheets with THACO done out for the PCs to see on their sheet and many people played with saying "I rolled an 11, I hit an AC of 3" which helps the DM adjudicate combat faster and can make play smoother.

Then 2e felt great with putting THACO and saves in the PH so they were expected part of PC knowledge and the DM would be less speed bumped whenever they came up.

#### Sacrosanct

##### Legend
The real problem with THAC0 is that the "easy" mode formula of die roll + AC >= THAC0 conflicts with player nature. When players roll an attack die, every player -- and I do mean every player -- wants to know "What AC did I hit?" That's the question they always pose. It's one of the most common questions asked at the AD&D table. Players can't measure in their performance *against the other PCs and NPCs* without doing that step.
This sounds like it might be a shock to you, but no, lots of players couldn't care less how much their PC did in power compared to the other players. Most people I gamed with since the early 80s had the idea that the game was a team sport, and who cares if Bob did 1 or 2 more damage per round as long as I had fun with my PC.

And no one asked, "What AC did I hit" because you knew what AC you hit as soon as you rolled. That's what THAC0 is, literally. You roll your attack dice, and subtract the result from what your THAC0 is. That simple. If my THAC0 was 18, and I rolled a 14, I knew I hit AC4 or worse. THAT'S what players called out in combat. Not "What AC did I hit?", but "I hit AC4" and the DM would reply "that hits" or "that's a miss."

THAC0, in it's "easy" mode, seeks to obscure that information from the player by design. Gygax assumed the player would never know their THAC0. .
Wait, what? Of course players knew their THAC0. It was right there on their character sheets. Like, literally right there! If players weren't meant to know their THAC0, the design team did a pretty bad job at it.

#### Sacrosanct

##### Legend
In case you're curious, these are the character sheets from pretty much the beginning: Moldvay's basic, and 1e (note how players knew what they needed to hit because there were sections for it on the sheets):

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#### Charlaquin

##### Goblin Queen
Wait, what? Of course players knew their THAC0. It was right there on their character sheets. Like, literally right there! If players weren't meant to know their THAC0, the design team did a pretty bad job at it.
I’ve observed that many players, especially players who started with 3e or later, conflate THAC0 with descending AC generally. I know it took me an embarrassingly long time to realize that THAC0 was not shorthand for descending AC, but in fact referred to a 2e evolution from AD&D’s Attack tables.

#### Hussar

##### Legend
Did you do it differently? I guess I never considered the possibility that the DM might just ask you to roll and tell him the number you got and your THAC0, do the math himself, and tell you if you hit or not. That sounds awful, it’s no wonder it developed such a bad reputation if folks were doing it that way.
I'd say yeah, that's typically the way it was done. Prior to Thaco, you'd have a matrix on the DM's screen, so, again, you call out your attack number, and the DM consults the matrix to tell you if you hit or not. AFAIK, that's typically the way it was done, so, virtually all the math was dumped on the DM to figure out.

Of course, after saying that, I realized that my group would generally just tell me what AC they hit. "17, that hit's an AC 3, did I hit?" was a pretty common phrase.

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