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12:35, 13th April 2024 (GMT+0)


Posted by V_V
member, 912 posts
Just like that...
my journey..finds me home
Thu 26 Mar 2020
at 12:06
  • msg #1


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
  • msg #2


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 2020.
member, 149 posts
joined 6/27/2002
Thu 26 Mar 2020
at 20:05
  • msg #3


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
  • msg #4


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
  • msg #5


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
  • msg #6


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 2020.
Aleph Null
member, 40 posts
I have my PhD
In Wumbology
Tue 29 Sep 2020
at 13:59
  • msg #7


In reply to Rystefn (msg # 4):

This is mostly correct, except the ORIGINAL original system actually went from 2 to 9

But yeah, the idea of descending armor class was that "first class armor" is meant to be better than the others...except then you have 0-class armor and all the way to negative 10, so that sort of breaks down...THAC0 was technically just a simplification of the OD&D combat tables where you could actually calculate what AC you hit without having to look at a table. But it's not strictly equivalent, because it turns out that the BECMI tables got weird at high levels... once you got below THAC0 5, you started getting bonuses to damage against targets with poor AC. And since your attack roll to-hit-armor-class was post-modifiers, this actually mattered. Take a 19th level fighter (THAC0 7) with a modifier of +6 to hit (from weapon mastery; this was not all too uncommon at that level). If you look at the to-hit-AC table, you can hit an armor class of -14 through -18 on a roll of 20, which because of your modifier is actually a roll of 14...and a roll of 15 would hit AC -19. So if you were using the tables, you actually had to look it up, because a roll of 14 here hits an AC of -14, while a roll of 15 hits an AC of -19. If you wanted to do the math, you could take into account the 5 repetitions of "20" on the table...but that's a pain in the neck.

TL;DR: even though people give THAC0 a bad rap, it's actually a simplification of the messy combat tables that preceded it.
Sir Swindle
member, 252 posts
Tue 29 Sep 2020
at 14:19
  • msg #8


Essentially if you use THAC0 you only need to look at 1 modifier before rolling (the target's armor class).

Godbound essentially uses THAC0 without saying it. They say "Your roll plus your attack modifiers plus your target's AC must equal greater than 20 in order to hit."

Which means the roll you need to hit AC0 (your THAC0) is 20-your hit modifiers. If I'm +5 better at hitting then my THAC0 is 15 and I better hope I'm fighting things with AC4+. If I'm +10 Better at hitting then my THAC0 is 10 and an AC0 enemy is reasonably approachable.

In 3.5 terms you could do the same thing and just instead of an attack bonus you could write down your THAC20. It would just be a little less convenient because no one is in the habit of interacting with that and your DM would just be confused if you told him you rolled 5 over your THAC20.
member, 543 posts
Thu 29 Oct 2020
at 04:49
  • msg #9


THACO is fun. THACO is physics. THACO is far simpler than it seems.

  First look at your sheet for this example we will use a 10th level human fighter with negligable strength(meaning no modifiers again a 10 IIRC).

  a 10th level fighter has a THACO of 10. Now that basically translates into a +10 attack rating. Why is that important because for every AC Higher that zero it adds that number to the attack rating. SO an AC of 9 means you have a +19 modifier meaning in order to hit you can't roll a 1. AND as another feature an AC of -6 means you attack modifier is now a +4 meaning you need a 16 or better to hit that target. THACO was part of the reason that the younger folks disliked 2nd edition.

  The third edition and later mechanic and indeed as others have pointed out many games grew out of this. Third Edition just simplified the math. AC was now a positive number alone and was the number you had to beat now instead of reaching 20. Depending on the build you could get a THACO of 2 at 3rd level with an AC of -3. And yes it is a legal build just super specific and requires set up.
member, 978 posts
Umm.. yep.
So, there's this door...
Thu 16 Feb 2023
at 18:47
  • msg #10


Fast Summary:

"Back in the Day", it was the norm for the GM to handle all actual dice rolls, or at least most.  A GM wouldn't tell the players what the armor class was - thus the need for a hyper simplistic means of performing the calculation.

GM: "what's your THAC0"
Grog: "12"
GM: looks at AC, it's 5.  12-5 = 7 or higher to hit on a d20.
GM: drops a d20, "you hit!"
Larson Gates
member, 39 posts
Thu 16 Feb 2023
at 19:53
  • msg #11


Fast Summary:

"Back in the Day", it was the norm for the GM to handle all actual dice rolls, or at least most.  A GM wouldn't tell the players what the armor class was - thus the need for a hyper simplistic means of performing the calculation.

really day imaginary day was that? I've never either played in a game or run a game in 30 years when the GM rolled combat rolls on behalf of the players.
member, 2 posts
Fri 17 Mar 2023
at 12:56
  • msg #12


In reply to Larson Gates (msg # 11):

Before 3rd edition. It was probably coming out of favor near the end of second.
supporter, 768 posts
Fri 17 Mar 2023
at 14:58
  • msg #13


I've been playing D&D since 1978, and I have never been in a game where the GM rolled for the players.  It was and is just a GM preference.  Don't confuse your personal experience with general trends.
moderator, 16164 posts
He's big, he's bad,
but mostly he's Ron.
Fri 17 Mar 2023
at 15:19


In reply to Zag24 (msg # 13):

Same here.  Started with the old white box set.  Never had a GM roll dice for players, in any edition of the game, and with a variety of GMs.
member, 16 posts
Fri 17 Mar 2023
at 20:39
  • msg #15


I've been in games where the GM would occasionally roll for the players, but only for things where neither the player nor their character should know the outcome. Obviously a GM announcing "everyone roll perception" tips everyone off to there being a trap or ambush nearby even if they fail their roll.
member, 33 posts
Suffering for her art
Sat 18 Mar 2023
at 00:33
  • msg #16


Started in '79. Did this rolling on the player's behalf when it made sense, and had them roll  at random moments as well esp for perception and stealth. They could roll when they were doing something actively like sneaking past a guard or searching a room.

But once we got started in a fight and the THACO was obviously known the players could roll everything. Sometimes I'd let a player roll for the NPC that was attacking them. Saved me time.
member, 20 posts
GM & Player
always in worlds away
Sat 18 Mar 2023
at 02:45
  • msg #17


"everyone roll perception" tips everyone off to there being a trap or ambush nearby even if they fail their roll.

Actually I do this randomly, in my games. It keeps them on their toes. Its also something that allows me to toss something in I want them to see or find.
member, 17 posts
Sat 18 Mar 2023
at 22:30
  • msg #18


In reply to moonbunny (msg # 17):

If you're running a game with random encounters it's easy to disguise secret perception rolls. Otherwise you can always just occasionally throw the dice for no reason.
Larson Gates
member, 41 posts
Sat 18 Mar 2023
at 23:19
  • msg #19


Technically the players don't need to know what the THACO is.. they just roll the dice and add what ever modifiers they have. You just tell them whether they hit or not. They may then be able to work out what the AC of the opponent is and therefore whether they would have hit or not. But that's completely different to you as GM making combat rolls on their behalf.
I've also used the 'roll randomly' for no apparent reason, and the make 10 rolls at the start of the session, which I then record, and tick off as a player uses them up.
member, 1056 posts
Event: Arrival
Horizon: May 5th
Sat 6 May 2023
at 01:42
  • msg #20


I completely missed the recent surge of discussion here! I've been homeless from February 2022 to April 2023, and while I had web access I didn't expect this thread to come alive.

Nonetheless, I think the topic of GMs rolling for players is pretty much a regional thing. By regional, I mean that your groups were found by word of mouth and social units formed unlike they are now.

Players I met, from all systems, vary; so as was said personal experience is just that. I will amend that it is a trend, but it's a trend like fashion; on a local scale some circles and intersecting circles have that trend, because they're part of the same network. The totality of what is and isn't is a zeitgeist unknown.

I will close this this by saying, this has drifted from THAC0 to dice rolling etiquette. Please close this thread, unless it's deemed relevant here.
This message was last edited by the user at 02:02, Sat 06 May 2023.
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