Thac0

[HamsterKun]

Being a 5e main, I decided to try out AD&D 2e for the sake of it. If there’s one mechanic I still don’t understand, it’s THAC0. Can anyone explain it for me?

[Thrudd]

It means "to hit AC 0". The lower your number the better- AC is descending instead of ascending. You just fill in the chart from there: if thac0 is 20, then to hit ac 1 you need 19, 2ac -18, etc.
The other way is to subtract the AC you're trying to hit from your THAC0- that's the number you need to roll to hit.

There's nothing else to it- it's describing the same thing as 3e bab.

[MeeposFire]

The other option is to just do subtraction. Take the THAC0 and subtract your roll.

your THAC0 is 15 and you roll a 17. 15-17=-2 so you can hit up to a -2 AC or worse (worse in this case is a higher number) with that roll and that THAC0.

Or with the same THAC0 (15) and I roll a 12 I hit an AC of 3 or higher (15-12=3).

Personally subtraction works best for me but not for everybody else. I find the other methods more confusing but others find them less confusing.

[Kurald Galain]

Being a 5e main, I decided to try out AD&D 2e for the sake of it. If there’s one mechanic I still don’t understand, it’s THAC0. Can anyone explain it for me?

Subtract the enemy's AC from your THAC0. You need to roll that number (or higher) to hit.

[Kurgan]

It has been a long time since I played 2nd edition adnd, but I vaguely remember simple addition worked: Roll + Armor >= thac0 is a hit.

Your thanc0 is a 19, you are attacking an enemy with AC 7, you roll a 12.

12 + 7 = 19, you hit.

You roll a 3, 3 + 7 = 10, which is less than your thac0 of 19, so you miss.

In terms of later editions, think of your Thac0 as your target number and the enemy's armor class as the modifier, instead of the other way around.

[LibraryOgre]

Being a 5e main, I decided to try out AD&D 2e for the sake of it. If there’s one mechanic I still don’t understand, it’s THAC0. Can anyone explain it for me?

One thing I always did was just doodle the numbers I hit in the margins. Like, my ThAC0 is 18, and I have a +2 to hit with a given weapon? I'd write

20 -4
19 -3
18 -2
17 -1
16 0
15 1

And that was usually enough to tell me what I hit based on my die roll. I prefer knowing what AC I hit, because the DM wouldn't always tell me what AC the thing was for the AC-based calculations.

[lightningcat]

I mostly used the old Masque of the Red Death sheets, which had a spot for the THAC0 calculations already done.

[Particle_Man]

One thing I always did was just doodle the numbers I hit in the margins. Like, my ThAC0 is 18, and I have a +2 to hit with a given weapon? I'd write

20 -4
19 -3
18 -2
17 -1
16 0
15 1

And that was usually enough to tell me what I hit based on my die roll. I prefer knowing what AC I hit, because the DM wouldn't always tell me what AC the thing was for the AC-based calculations.

That reminds me of the 1st edition tables and chartes from the DMG or DM screen!

[LibraryOgre]

That reminds me of the 1st edition tables and chartes from the DMG or DM screen!

Pretty similar, yeah. But it can be handled in just a few numbers, and doesn't have the fluctuations that the matricies had.

[Greywander]

I learned THAC0 from the Baldur's Gate video game, but it's been a long time since I played it. I remember it wasn't actually that hard once you figured it out.

IIRC, THAC0 is basically the DC of your attack roll. The enemy's AC is a bonus to your attack roll. So if your THAC0 is, say, 18, that means you need to roll an 18 or higher to hit. If the enemy has an AC of 3, that means you get a +3 bonus to your attack roll, and thus only need to roll a 15. Really good AC actually goes negative, becoming a penalty instead of a bonus.

I could be misremembering, though, so maybe someone better versed in D&D can chime in and correct me.

And yeah, it feels so backward and strange. That's probably why they got rid of it. Mathematically, I don't think anything has really changed, it's just that the math is being expressed differently now in ways that make more intuitive sense, i.e. higher numbers are always better now, whereas before you wanted both low THAC0 and low AC.

[LibraryOgre]

IIRC, THAC0 is basically the DC of your attack roll. The enemy's AC is a bonus to your attack roll. So if your THAC0 is, say, 18, that means you need to roll an 18 or higher to hit. If the enemy has an AC of 3, that means you get a +3 bonus to your attack roll, and thus only need to roll a 15. Really good AC actually goes negative, becoming a penalty instead of a bonus.

I could be misremembering, though, so maybe someone better versed in D&D can chime in and correct me.

Nope, that works out. Good way of putting it, too.

[Zakhara]

THAC0 internalizes your Target Number To-Hit instead of making it based on the enemy.

It synergizes with descending AC because AC is a modifier supplied to the Attack roll.

Roll d20, add enemy AC. If it meets/exceeds your THAC0, you hit.

Your THAC0 lowers based on your class/level (which is a good thing).

(I find this much, much more easily-understood than the subtraction method; humans are generally better with addition)

[opaopajr]

It's just your to-hit bonus (BAB) in reverse, counting down from 20.

The other option is to just do subtraction. Take the THAC0 and subtract your roll.

your THAC0 is 15 and you roll a 17. 15-17=-2 so you can hit up to a -2 AC or worse (worse in this case is a higher number) with that roll and that THAC0.

Or with the same THAC0 (15) and I roll a 12 I hit an AC of 3 or higher (15-12=3).

Personally subtraction works best for me but not for everybody else. I find the other methods more confusing but others find them less confusing.

Yup. And it is easy to use if you think about it as a DC.

The main benefit is you can HIDE the AC of new monsters (sorta like just reading WotC to-hit) so it's more DM-facing, less player-facing knowledge.

So THAC0 20, roll 11, have weapon spec of +1 to-hit, and a +2 magic dagger: just add up your roll, spec, & magic weapon and check the difference vs. your THAC0 (attack DC). So 11+1+2 = 14. Difference between that and THAC0 20 is 6, you can hit AC 6 and higher.

(I usually just convert THAC0 into BAB anyway, though. This way I assume THAC0 20 (like DC 20) and let the negative ACs read as a penalty (otherwise AC reads as a bonus to-hit). Then I let players add up player-facing bonuses while I worry about penalties and behind-the-scenes DM-facing stuff. Finally I check vs. "DC 20."

e.g.
THAC0 17 (BAB +3), Die Roll 15, STR bonus +2, Weapon Spec +1, Bless +1, Magic Weapon +2, Called Shot -4, vs. AC -2.

Player just adds all their known bonuses to the die and calls that out to me: 15 +3(bab) +2 str +1 ws +1 bless +2 mw. To-hit 24.

I tabulate the penalties and secret stuuf, like AC. -4 + -2 = -6.

Total the subtotals up and check vs. "DC 20" = 18, miss. Would have hit without the Called Shot, but the player does not know that yet. The mystery continues...

Yes, there could be a lot of modifiers. But if you kept it focused to what players need to worry about vs. what DM needs to worry about you make the task smaller.)

[oxybe]

The other option is to just do subtraction. Take the THAC0 and subtract your roll.

your THAC0 is 15 and you roll a 17. 15-17=-2 so you can hit up to a -2 AC or worse (worse in this case is a higher number) with that roll and that THAC0.

Or with the same THAC0 (15) and I roll a 12 I hit an AC of 3 or higher (15-12=3).

Personally subtraction works best for me but not for everybody else. I find the other methods more confusing but others find them less confusing.

When I ran a 2e Ravenloft one-shot a while back, this is exactly how I explained ThAC0: sub your (modified) roll from your ThAC0 and tell me the number, that's the AC you can hit. I'll do the rest.

after one warm up fight, it had clicked and the group was good to go.

[hamlet]

It has been a long time since I played 2nd edition adnd, but I vaguely remember simple addition worked: Roll + Armor >= thac0 is a hit.

Your thanc0 is a 19, you are attacking an enemy with AC 7, you roll a 12.

12 + 7 = 19, you hit.

You roll a 3, 3 + 7 = 10, which is less than your thac0 of 19, so you miss.

In terms of later editions, think of your Thac0 as your target number and the enemy's armor class as the modifier, instead of the other way around.

This is probably the easiest way short of building a chart for yourself. Character's to hit roll and DM adds target AC and compares to THAC0 greater than or equal hits. It slows you down but not nearly as much as trying to figure subtraction etc or checking a chart really.

Plus, if you don't care that much, you can just say "yeah that oughta hit" when they roll high enough for your sake and just fudge it. Works best for low level stuff.

[BigBadHarve]

Forgive any overlap with above answers - As a DM I don't like to tip off the exact AC of the monsters/enemies to the players.

They get to know AC 'type' if the opponent is armoured, of course, but the final AC is a mystery.

Ie: Facing a warrior with Chain and Shield (AC type 4) but also had a dex bonus of +2, meaning his final AC was 2. A player rolled and hit enough to hit AC 4, which they assumed would connect, but ultimately didn't. So they knew he had some other protection in play, either magic, or Dex etc.

This being the situation, I have my players simply roll to strike and subtract the strike roll from their THAC0, the result being the best AC they are able to hit.

IE: THAC0 is 19, Player rolls a 16, (19 - 16 = 3) they can therefore hit anything that is AC3 to AC10.

It works the other way around as well, when AC values are known, you can just subtract the AC from the THAC0 to determine the roll needed to hit.

IE: THAC0 is 19, and the opponent has AC6, (19 - 6 = 13) meaning the attacker needs to roll a 13 or higher to strike.


On a personal note, I had no problem with later editions using ascending values for the attack rolls, my issue is that they stopped the range parameters of the earlier system so the escalated numbers got a touch ridiculous.

In 1st and 2nd Ed, the AC range goes from AC 10 down to AC -10. These are the capped ranges.

It would have worked well in 3rd ed (I'm not familiar enough with 4th or 5th that I know if they fixed this or not) if they'd just reversed the integers... made the range from AC 10 - AC30, but kept everything else similar.

Instead, I gave up after seeing creatures with AC 50... My +1 magic weapon was soooo useful...

[Khedrac]

In 1st and 2nd Ed, the AC range goes from AC 10 down to AC -10. These are the capped ranges.

Well, -10 was the cap for 1st Ed, but 2nd Ed removed it (well made it a soft cap) - a number of dragons reached AC-11 in the first Monstrous Compendium and other things followed later (but they were pretty rare).


As for the amount of maths involved, THACO does seem more complex, but if you play a melee type in 3rd Ed the maths for keeping track of power attack and bonuses from allies quickly becomes quite a lot harder - I do not recommend a 3.5 Fighter for anyone not good at mental arithmetic. Most of my friends use as much power attack as they have boosts to hit - just to avoid recalculating attack rolls.

[MeeposFire]

Yea the AC cap changed depending on which version of AD&D you played or even which campaign setting. Some had no cap at all on AC.

[BigBadHarve]

Yea the AC cap changed depending on which version of AD&D you played or even which campaign setting. Some had no cap at all on AC.

I think my comparative point to 3rd Ed stands - in that while 2nd Ed may have had the occasional exception to the cap (I think -12 AC is the most extreme case) that's a rare circumstance, where in 3rd Ed an AC of well over 30 or 40 was fairly common and just kept going. As with hit points, which could often number into the hundreds.

It's the rampant escalation that bothered me. Even 5th edition suffers from this. Flipping through the Monster Manual I noticed Ogres had an average of 60 hp... other creatures were more ridiculous.

Meanwhile weapons are still down in the 1-6 and 1-8 range.

Maintaining a set of parameters as well as a common 'average' is the way to go, in my opinion.

[SimperingToad]

Actually, there was no 'cap' in 1E, though certainly no one that I have played in the game with ever experienced anything outside the 10 to -10 range shown on the tables, so it wasn't an issue.

Armor class below 10 is not possible except through cursed items. Armor class above 2 is easily possible due to magical bonuses and dexterity bonuses. To determine a "to hit" number not on the charts, project upwards by 1's (5% increments), repeating 20 six times before continuing with 21 (cf. MATRIX I.A.).

So, it is possible to have an AC worse than 10 through cursed magic items. And, the paragraph does mention that it is also possible to go beyond -10 and gives the method to continue the number progression. But, as I said, I've never personally seen either extreme happen.

[opaopajr]

It's the rampant escalation that bothered me. Even 5th edition suffers from this. Flipping through the Monster Manual I noticed Ogres had an average of 60 hp... other creatures were more ridiculous.

Meanwhile weapons are still down in the 1-6 and 1-8 range.

Maintaining a set of parameters as well as a common 'average' is the way to go, in my opinion.

I get what you are saying. Thankfully 5e tries to return to Bounded Accuracy. So you don't see 3e or 4e treadmill level increases to ACs, BABs, and Saves.

However they also changed their paradigm from acceptable whiffing in TSR D&D to hitting more frequently. This was done as a nod to human psychology liking things within the magic 65~75% success range to feel "capable yet challenged." So to compensate for that they bloat HP instead.

Not as bad as previous WotC attempts, but it is a noticeable aesthetic shift.

I too personally prefer the old TSR aesthetics more. I found combats could end very fast so we could get to the other pillars of fun: exploration and social. Plenty of whiffing, a few good licks, battle's over; one could easily get a handful of encounters in an evening with plenty of shenanigans along the way, possibly two handfuls. But compared to the their other editions, I like where WotC is going for once.

[OlegRU]

Once you get it - it becomes second nature. The reason so many people don't get it is because it sounds like one is contradicting logic when explaining it.

It's your attack dice
To Hit Armor Class 0

What is the number you need to roll greater or equal to in order to hit a 0 armor class.

Keep in mind that AC was the lower the better in the old DND (started with 10 AC and I think it went to -10 at best for people)

You would subract AC from the THAC0 - let's say 18 THAC0 you're attacking an AC 0 - you subtract 0 from 18 and get 18 - you need to roll 18 or higher to hit.

Then, of course, it scales - with a THAC0 of 18, attacking an AC -1: 18 - -1 (subtracting a negative is adding in math), so you need a 19 or higher to hit.


I've met a DM who still runs 1e at oneshots and he just basically gives people charts as part of their pregen characters so they could look it up without explanation.




I gotta read over this later - quite tired and hope I wrote this making sense lol.

[The Kool]

You would subract AC from the THAC0

Or do it the way I always have: Add their AC to your roll. Your target number is your THAC0, you add your weapon bonuses if you have any and then add your target's AC. Did you hit your THAC0? Then you hit.

[D+1]

Actually, there was no 'cap' in 1E, though certainly no one that I have played in the game with ever experienced anything outside the 10 to -10 range shown on the tables, so it wasn't an issue.

Armor class below 10 is not possible except through cursed items. Armor class above 2 is easily possible due to magical bonuses and dexterity bonuses. To determine a "to hit" number not on the charts, project upwards by 1's (5% increments), repeating 20 six times before continuing with 21 (cf. MATRIX I.A.).

So, it is possible to have an AC worse than 10 through cursed magic items. And, the paragraph does mention that it is also possible to go beyond -10 and gives the method to continue the number progression. But, as I said, I've never personally seen either extreme happen.

But people also don't appreciate just how the attack matrix in 1E WORKS. It isn't as straightforward as people assume.

By default in 1E a 20 is the best to-hit score you can get. No matter what your stacks of bonuses are, a result of 20 is your limit. Consulting the table, that can prevent you from hitting the really REALLY low AC monsters. The OPTION (DMG p.82) makes it harder still. With the optional rule, after that first 20 listed (where you get to add all the bonuses you can muster in order to GET that maximum result of 20) you instead need a NATURAL die result of 20. All your bonuses, no matter what they are and where they came from, then don't matter. If you didn't roll a natural 20, YOU DIDN'T HIT. When the table gets to results of 21 and above, beyond the repeated 20's, you STILL need a natural 20, but then you also need bonuses to add to that. That's really a corner case though. If you're attacking a very low AC opponent (somewhere in the negatives) in the first place, chances are very good that you have a +1 from SOMETHING - strength, magic weapon, etc. So, even though you still need that natural 20, it's unlikely that you wouldn't also have the additional bonus to add to it.

The 1E MM doesn't even have a monster better than AC -8. Using the matrix in the DEFAULT manner, a 5th level fighter can hit that -8 AC while applying all the bonuses he can muster to bump the to-hit result up to 20. And all the repeated 20's means that he needs the same total of 20 to hit AC-8 as he does to hit AC-4, and the more to-hit bonuses he comes up with the better his chance of hitting anything in that AC range. Using the OPTION, he just has a 5% chance to hit, period. That's not a matter of being UN-hittable, it's a matter of how likely that AC-8 monster to BE hittable - just a flat 5%, or with all the bonuses you can muster. But an AC -10 opponent is flat-out not hittable by a 5th level fighter when using the default 1E rules and matrices. Only if you're using the optional rules from DMG p.82 could a 5th level fighter hit AC-10 - and they'd need a natural 20 and a +1 to-hit bonus from something in order to do that.

THAT's the effect that the repeated 20 actually has - that even when PC's are advancing in levels, it isn't that they CAN'T hit certain AC's so much as that their chance to hit is FORCIBLY limited to either a max to-hit result of 20 by default, or using the optional rule that puts limits on even higher level PC's with lots of awesome gear by invalidating their plentiful to-hit bonuses against such opponents.

When you use THACO rather than the 1E combat matrices, you actually push more very low AC's out of reach of low-level PC's because they simply won't have the plentiful bonuses they will need to get results better than 20. For higher level characters using thaco, they may have those piles of bonuses from magic weapons, strength, etc. but it then factors in far MORE than it would if you were using the matrices as 1E intends them to be used. So, comparing 1E and 2E; with 2E the PC's at low levels won't be able to hit very-low AC opponents (but then they won't face many such opponents, if any), and at higher levels they will hit opponents a lot more frequently, even low-AC opponents, because they aren't artificially being prevented from applying all those bonuses.

People overwhelmingly assume in 1E that your bonuses always fully combine with the die result and push that combined result higher - but in 1E it's NOT supposed to work that way and that can make a huge difference in how the game plays.

[Ashtagon]

1d20 + your class level bonus + the target's AC.

A result of 21+ is a hit.