member, 912 posts
 Just like that...
 my journey..finds me home
Thu 26 Mar 2020
at 12:06
Would someone just explain the very base idea of THAC0. I have a few bad experiences, that all conflicted as to how it worked. I am led to believe one thing, but I just don't know anymore.

I lost the few 2e TSR (IIRC) books I had, in a fire a little over a ear ago. I'm hoping I can find some legal pdfs shops that sell them. Until then, I can't just "read the manual".

I'm really interested in just knowing this very elementary mechanic that has never been explained to me properly. From there, I may have more questions.
 subscriber, 105 posts
Thu 26 Mar 2020
at 12:42
Oh man you're making me dust off my memories here.

All right, the clue is in the Acronym.  "To Hit Armor Class Zero".  Let me start with the very basic scenario.

Let's say a level 5 Baker has a THACO of 18 is fighting an opponent with Armor Class (AC) 0.

The Baker's player wants to make an attack.  The player rolls a 20 sided die.  If the Baker rolls an 18 or better, the attack is successful and he is allowed to roll damage.

So first thing to remember, higher d20 rolls = good for the attacker and bad for the defender (Edit, hilarious misspelling of "defender" as "attender" lol).

Where this gets screwy is the way AC worked in 2E, smaller (ideally negative numbers) is better.

Second thing to remember is lower AC values, which can go negative are good for the defender and bad for the attacker.

Where it gets screwy is when you have to start doing the math to figure out when our Baker hits different armor classes besides zero.  The arithmetic is trivial, the logic of how this works however is counter intuitive.  Also, remember if you're not someone who does math every day than minus a negative is a plus.

Going back to our level 5 Baker with his THAC0 of 18.  To hit his opponent with an armor class of zero, he needs to roll an 18 on his d20 when attacking.

From here, we can figure out the permutations by subtracting the armor class he wants to hit from his THAC0.  That's what he must roll.

What if the armor class he has to hit is negative one (-1)?

THAC0 18
Armor Class (-1)
Arithmethic 18-(-1)=18+1=19

Our Baker with a THAC0 of 18 must roll a 19 or higher to hit AC -1.

What if the armor class he has to hit is six (6)?

THAC0 18
Armor Class 6
Arithmethic 18-6=12

Our Baker with a THAC0 of 18 must roll a 12 or higher to hit AC 6.

That's the basic high level idea.  It's screwy and there's a reason modern games don't use it any more.  Mathematically it's not that complicated really but again it's counter-intuitive.  It gets really hairy when you have to start remembering that pluses to hit are actually lowering your THAC0.

This message was last edited by the user at 12:43, Thu 26 Mar.

 member, 149 posts
 joined 6/27/2002
Thu 26 Mar 2020
at 20:05
TAHC0 is what you have to roll on the D20 to hit an AC of 0
 member, 52 posts
Mon 30 Mar 2020
at 19:07
The problem with Thac0 is, and always has been, that it's explained poorly. It's literally the exact same system all later versions of D&D use, except counting the other way. The math is not harder. It's still just counting.

You roll the die. You add or subtract your relevant modifiers. You look at your Thac0. You count the difference between the two numbers to figure out which AC you hit. If you rolled higher than your Thac0, you hit a negative AC.

If you prefer having it explained as math, then: Thac0 - die roll = AC you hit.
That's it. You just subtract instead of add, and it's literally exactly the same as the 3, 4, and 5 attack bonus system.

- You have Thac0 18. You have +2 to hit. You roll a d20. If you roll a 10, you add your +2 to get 12. You count the difference between 18 and 12. You hit AC6. (18 - 12 = 6)
- You have Thac0 15. You have -1 to hit. You a 19, subtract the one to get 18. You count the difference between 15 and 18. You hit AC-3. (15 - 18 = -3)

If you prefer, you can take your +2 to hit and apply it to your Thac0 instead. To do this, you add a negative and subtract a positive, which is where some people get lost. In the first example, above, this gives you a "modified Thac0" of 16. You roll a 10. You count the difference. You hit AC6.

People who get confused about lower armor being better: Think of it in the way it was written. It's not an "Armor Rating." It's an "Armor Class." If you have Armor Class 1, then you have First Class Armor. If you have Armor Class 3, you have Third Class Armor. Which sounds better, First Class Armor or Third Class Armor? First, right? That's why AC1 is better than AC3. Zero and negative numbers are later additions that were tacked onto a system that originally only went 1-10.
 member, 147 posts
Tue 14 Apr 2020
at 01:19
I'll be honest, every game I had Post-HS I had a DM's screen so it was never a problem "figuring it out". And Yes, 1st Ed had thaco as well, just slightly different.
 member, 1 post
Tue 28 Apr 2020
at 08:55
Everything in the other posts was correct, but here is the short version. Sometimes keeping it simple helps.

Subtract the targets AC from your thac0. You need to meet or beat that number with your attack roll. That's it. And remember that subtracting a negative number means adding

thac0 15 against AC -2. Roll a 17 or higher after any modifiers and you hit.

This message was last edited by the user at 09:00, Tue 28 Apr.