Let's Talk About THAC0

Harzel · Jun 8, 2019

While I knew what THAC0 was, and I'm pretty sure I used it, and I seem to recall embracing it as an improvement over the tables, my memory is much too sketchy to remember exactly what the protocol was for players telling he what they had rolled. All that said, I would be astonished if anyone outside of a few geezers on ENWorld thought that BaB was not a vast improvement.

Of course, without a lot of research that no one would ever fund, it's not possible to be sure whether or why some people might have trouble wrapping their heads around THAC0. However, I will venture a guess as to why it has the 'reputation' of being 'complex' despite involving more or less the same arithmetic as BaB. First, I would guess that 'obscure' would be a better description of some people's negative take on it than 'complex'. Second, I would guess that this is a good example of an aspect of earlier D&D that was a barrier to entry for non-nerds. (Sure, there were lots of folks for whom THAC0 was 'no problem', the vast majority of whom I would conjecture were folks who were very facile with quantitative and abstract processes.) The source of the obscurity of THAC0 (or at least a major contributor), I think, is that the THAC0 value is defined by a hypothetical: it's the value you would need to roll if your target had AC0. Intuitively, to me at least, it is notably more difficult to understand (more obscure) how that plays into the to-hit calculation than the quantities involved in BaB - the dice roll, your to-hit bonus, and the target's AC - which are all defined in obvious ways by and related in obvious ways to the current circumstances.

Maxperson · Jun 8, 2019

We never had issues with THAC0 in any of our 2e games over the years. The math was just too simple to do for us to have an issue with it.

Lanefan · Jun 8, 2019

Sacrosanct said:
It says "adjusted to hit Armor Class." That clearly means you put down what you need to roll to hit the various armor classes after adjustments (modifiers). Seemed pretty obvious to us anyway, back then. I mean, that's what the words mean.

Problem with that was - and still is - those adjustments can change back and forth fifteen times within a single session!

Someone casts Strength on you - change adjustments, and again when it wears off
You're firing a missile instead of swinging an axe - change adjustments (unless your Str and Dex just happen to give you the same to-hit bonus)
Your axe is +2, your bow is +1 and your sword is +0 but +3 vs. Evil - change adjustments every time you change weapon and sometimes when you change opponent
Someone casts Bless for you or Bane against you - change adjustments, and again when it wears off

Writting down the unadjusted base I can see, but rewriting it every time the adjustments change seems like massive overkill.

So if I needed a 19 to hit an armor class 0, and I had a +1 modifier for strength and a +2 modifier for the magic weapon (that's why each weapon has it's own table)

Ah, that helps a bit. The sheets I've seen just have the one row for THACx.

Now the adjustments - for each weapon! - might only change six times a session.

digitalelf · Jun 8, 2019

Harzel said:
So did THAC0 get introduced prior to 2e in one of the Basic versions, or perhaps in Dragon magazine? I didn't realize this was an anomaly in my history until this thread.

THAC0 was first introduced to AD&D (1st Edition) as a usable game mechanic in 1983 with the module "UK3: The Sentinel" as an optional rule.

It latter appeared as an "official" part of the 1st edition ruleset in 1986 with the publication of the "Dungeoneer's Survival Guide" hardbound.

The term THAC0 (as opposed to a usable game mechanic) is actually much much older, and can be found in the 1st edition DMG (published in 1979) on pages 196 - 215 in the "APPENDIX E: ALPHABETICAL MONSTER LISTING". But at that point, it was little more than just another stat. There was no real explanation on how to use it as an actual game mechanic.

Lanefan · Jun 8, 2019

Morrus said:
To be fair, nobody has carefully explained to me that subtraction is really hard yet.

Is isn't.

But I still never used THAC0, for two reasons - one philosophical and one practical.

Philosophical reason: both as player and DM I want the combat matrix (or later, BAB) to be DM-side info rather than player-side.

Practical reason: for me as DM it puts in an extra unnecessary step in figuring out whether an attack hits or not; that step in fact being that I have to start the calculation an entirely different way. The way I do it now the player gives me their roll after adjustments, I add the target's AC* and any other unknown-to-players factors* and if the result is 21+ it's a hit. If instead a player were to just tell me "I hit AC 4" I'd have to reverse-engineer how that number was arrived at (knowing that the player in theory doesn't know the combat matrix) and then do the same steps I already do.

* - can be + or - numbers, or 0.

digitalelf · Jun 8, 2019

Lanefan said:
If instead a player were to just tell me "I hit AC 4" I'd have to reverse-engineer how that number was arrived at (knowing that the player in theory doesn't know the combat matrix) and then do the same steps I already do.

Like you (I assume), I too do not like to just provide the AC of the PC's foes to my players.

However, that said, neither the Combat Matrices nor THAC0 keep the player from determining the AC of any opponent they face.

The way you do it, the player can easily determine the AC of their opponent by the process of elimination... "Well, a 14 did not hit, but a 15 does, so that orc's AC must be at least 3".

Which is exactly what the players do with the DM using THAC0... "Okay, I rolled a 14, which hit AC 4, but that missed... I now just rolled a 15, which hit. So that orc's AC must be 3".

Don't get me wrong. I don't care how you resolve combat in your games... My point is that you are not keeping any secrets from the players by using the Combat Matrices.

Players that wish to metagame in this fashion, will do so.

Harzel · Jun 8, 2019

digitalelf said:
THAC0 was first introduced to AD&D (1st Edition) as a usable game mechanic in 1983 with the module "UK3: The Sentinel" as an optional rule.

It latter appeared as an "official" part of the 1st edition ruleset in 1986 with the publication of the "Dungeoneer's Survival Guide" hardbound.

The term THAC0 (as opposed to a usable game mechanic) is actually much much older, and can be found in the 1st edition DMG (published in 1979) on pages 196 - 215 in the "APPENDIX E: ALPHABETICAL MONSTER LISTING". But at that point, it was little more than just another stat. There was no real explanation on how to use it as an actual game mechanic.

Ah, ok, thanks. That explains my monster descriptions. I guess I figured out how to use it myself. And when we can give XP again, I'll give you some.

digitalelf · Jun 8, 2019

Harzel said:
Ah, ok, thanks.

No problem, glad I could help.

Oofta · Jun 8, 2019

So am I understanding this right? That one of the "advantages" of THAC0 was that you had a character sheet where you pre-calculated what AC you hit based on your D20 roll? Which you could do with any system that uses a reasonably small range of numbers to determine whether you hit or not?

As far as subtracting being more difficult, it's the combination of adding, and subtracting that you need to do (assuming you need to adjust your pre-written numbers) and some people have a hard time subtracting into negatives. But it's probably like trying to explain color to my buddy who's color-blind. No matter how much I can try to explain the difference between red and green, he's just not going to get it. If you don't understand why adding all numbers is easier than adding and comparing positive numbers is more natural for most people you'll just have to accept that it is.

Morrus · Jun 8, 2019

Oofta said:
But it's probably like trying to explain color to my buddy who's color-blind. No matter how much I can try to explain the difference between red and green, he's just not going to get it. If you don't understand why adding all numbers is easier than adding and comparing positive numbers is more natural for most people you'll just have to accept that it is.

Wow?