+/- to THAC0. AD&D confusion.
theJoshFrost
Member Posts: 171
So, I've beaten the Baldur's Gate series, how ever, I went the entire saga having essentially no idea about what the base rules of the game were. For some reason, I never got a manual with the game, so really the only thing I knew about the game was that low AC was good. I had NO idea what THAC0 was, but I assumed it was sort of important.
Alright so let's see if I got this right, if you have a THAC0 if 16, and your were fighting someone with 6 AC, you'd need a 10 or lower to hit them, right? Or is it higher? I STILL DON'T KNOW.
Alright but that's beside the point. If a magic item gives you a +2 to THAC0, given the systems reliance on LOW numbers, that seems like a bad thing, right? I want my THAC0 to be lower, and if I get a +2 to my 16 THAC0, I'd have 18, and I don't want that! Do I?
Unless what they REALLY mean by +2 is it would make my THAC0 14 instead, and a -2 would give me 18. So... wat?
Alright so let's see if I got this right, if you have a THAC0 if 16, and your were fighting someone with 6 AC, you'd need a 10 or lower to hit them, right? Or is it higher? I STILL DON'T KNOW.
Alright but that's beside the point. If a magic item gives you a +2 to THAC0, given the systems reliance on LOW numbers, that seems like a bad thing, right? I want my THAC0 to be lower, and if I get a +2 to my 16 THAC0, I'd have 18, and I don't want that! Do I?
Unless what they REALLY mean by +2 is it would make my THAC0 14 instead, and a -2 would give me 18. So... wat?
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Items that add a + to Thac0 actually lower it. So A sword that adds +2 Thac0 actually would make your Thac0 score 2 points lower than it was (ie Thac0 10 becomes Thac0 8).
The whole lower is better but abilities that add a +1 to something actually lower it is indeed quite confusing. I'm guessing it is one of many reasons Thac0 was done away with in later iterations of DnD.
Essentially it's like your reasoning toward the end of your post, you want your thac0 to be as low as possible, but the terminology is bonus-oriented: if you get +1 to thac0 it means your thac0 is reduced by 1, like from 15 to 14.. i.e. it just got one better.
Likewise, if you get an AC bonus of two, it means your AC got reduced by two.. it's just more commonly called -2, whereas thac0 is not.
In your first example, you need a 10 or higher to hit them. On a roll of 16 you hit AC 0, on 15 you hit AC 1. on 14 you hit AC 2, etc... and on 10 you hit AC 6. If you roll 9 you hit AC 7, and miss them.
Edit: Actually let me rephrase that.. what you usually get +1 to is your hitroll, which isn't the same as your thac0. Afaik, when thac0 improves as you gain levels, the game will say that it gets reduced.
Let me try to break it down step by step (warning, massive wall of text incoming):
Without any defensive bonuses, penalties or armor the A(rmor) C(lass) of any creature is 10.
Without any offensive bonuses or penalties an attack is a straight d20 roll (1-20).
After the attack roll, you compare it to the target's AC. In this case, anything from a 10-20 is a hit.
Any bonus SUBTRACTS from the AC. (this is the part that makes no sense and why subsequent editions of D&D don't do things this way) Therefore a negative armor class is actually a good thing.
aside: Typically the armor bonus is considered first, then DEX, natural bonus, deflection bonus, magic bonus etc. In D&D you are allowed ONE bonus from each type. So if you have armor on that gives you base AC6 and bracers that gave you base AC2, your AC would be 2 because only the larger of the two bonuses would apply. In this case your AC would not change even if you removed your armor, as long as you kept the bracers on.
Basically, at the default (+)10 AC an attacker gets +10 on their roll to hit. If the AC was a very respectable -10 they would get -10 on their roll to hit you. If you were in the middle at 0 the attacker would get neither a plus nor a minus but would have to meet their threshold, their To Hit Armor Class 0 number.
Let's go back to my first example. Without any bonuses you needed a 10 to hit someone of AC 10. If their AC was 0 you would need a 20 to hit them. (This is the weird part where it becomes easy to get lost) To 'simplify' this you would compare the number to your THACO instead so you only need to make one comparison.
Without any bonuses, a character's THACO is 20. (You need a 10 to hit AC 10, thus a 20 to hit AC 0) Every attack bonus lowers your THACO. High STR, class levels (esp fighter and similar types), magic spells or magic weapons all make it easier to hit your target, resulting in a THACO less than 20. (Being cursed, having a very low STR etc will increase the THACO). As the game goes on, any non-magic user should end up with a negative THACO. This means that if a target has an armor class of 0 it is impossible to miss them except on a roll of 1 (which always misses).
To use your example, if you had a THACO of 16 you would need a 16 to hit AC 0. If the target had AC 6, that would be a bonus to your result, so the number you would need to roll would be 10, a 55% chance (you need a 16 to hit 0, but his 6 AC means you need 6 less than that to hit, so 16-6=10). If his AC was -6 instead it would be (16+6=22). You can't roll 22 on a d20, but a roll of 20 automatically hits, so you have a 5% chance.
Since you are using a d20 to hit, you can simplify things by thinking of it this way. Any bonus to your AC means that there is an additional 5% chance per point of AC that any attack will miss you. Any bonus to your THACO means that there is an additional 5% chance per point that your attack will hit. Because of wonky AD&D rules, very negative numbers for both AC and THACO are good.
I will meditate upon this information, and perhaps when BG:EE finally releases, I will comprehend it. Thank you for the explanation.
Let's say you have a THAC0 of 13 and you roll a 17.
13-17 = -4.
So you hit anything that has an AC of -4 or worse (-3,-2,-1,...,5,7, etc.). If your target has an AC of -5, you miss. Remember: In 2ed, the lower the armor value, the better it is.
In other words: You want your THAC0 to be really low, but your rolls should still be high.
However, when it says a +6 bonus for chromatic orb that actually adds to the target's roll making the save easier. A penalty means that a number is subtracted from the saving throw, making a failure more likely.
A d20 is a die with 20 sides, numbered 1-20. There is a 5% chance of any given number in that range. Whenever someone makes an attack/tries to make a save in-game, the game generates a random number 1-20, applies bonuses/penalties and determines success or failure. There is an option under setting so that you can actually see the math in the text box every time an attack is rolled. I think it's called 'show to-hit rolls'.
No matter what you will always have at least a 5% chance to succeed (on a 20) and a 5% chance to fail (on a 1).
THAC0: 14
Bonus to hit from strength: +1
Weapon: mace +2
Opponent's AC: 5
Required roll(unmodified): 14 (THAC0) - 5 (AC) = 9 or higher on a d20.
Roll result: 7.
Modified Roll result: 7 (the roll) +1 (strength) +2 (magic bonus) = 10.
You hit.
To expand on what @ScarsUnseen said:
Your THAC0 is the target number. any attack you roll is attempting to roll higher than this number, so a lower THAC0 is better.
Your opponent's AC is added to your attack roll to determine a hit or a miss. You also get bonuses from magic weapons, high strength scores, or certain spells or abilities.
When you get a bonus to-hit, it's modifying the attack roll, not your base THAC0. However, it's simpler from a video game UI perspective to just roll that bonus into the THAC0, which is why it's represented as a subtraction.
To sum up:
THAC0: the base number that you have to beat with your modified to-hit roll in order to hit an opponent with an AC of zero.
AC: a bonus (if positive) or penalty (if negative) to the attacker's to-hit roll.
Example:
1 - Critical miss (always a miss)
20 - Critical hit (always a hit)
This means that a charmed squirrel can kill anything that doesn't fight back and isn't immune to physical damage.
Long story short it got mad and ran inside the warriors helm to attack his face.
Fear the squirrel!
Basically, Baldur's Gate will tell you two things about your Thac0.
Your BASE thac0 and your ACTUAL thac0.
BASE is your unmodified Thac0, determined ONLY by level of your char.
The number below, your ACTUAL thac0, takes into concideration your strength/dex (depending on melee or ranged weapon), your skill with said weapon and the enchantment of the weapon.
So your BASE Thac0 may well be 15.
Where as with the Skill, Str/Dex bonus and enchantment of the weapon could bring the ACTUAL thac0 to let's say.. 9, for example.
Which means that the number 15, your BASE thac0, means bugger all in combat. It's your ACTUAL thac0, that will determine your chance to hit + the 1D20 you always roll.
ACTUAL Thac0 9 + a roll of 10, means you hit ANYTHING with -1 AC or worse, but nothing with -2 or better. (Keep in mind, even if you HIT, the enemy may be immune to your weapon in which case Minsc is -usually- right. "No Effect!? I need bigger sword..")
I hope that was simpler and easier to understand.
Can only roll 1-20, so if the number is outside that range, cant hit(or miss) (unless its a 1 or 20)
Example: Bear has AC1, we have thac0 7
Need to roll (7-1=6) anything from 6 to 20 to hit that bear.
Here it is: HOW ATTACK ROLLS WORK IN 2 SIMPLE LINES
1.) Your THAC0 is the number you need to roll or higher on a 20 sided die.
2.) The opponent's AC is the bonus you get to your roll.
Say you attack an AC 4 with a THAC0 of 15. This means that you need to roll a 15 or higher to hit. You get a +4 to your roll. Thus, a roll of 11 actually hits (11 + 4 = 15 --> 15 or higher: HIT).
A negative AC is a "negative bonus" or, simply, a penalty. Say you attack an AC -1 with a THAC0 of 15. This means that you need to roll a 15 or higher to hit. You get a -1 to your roll. Thus, a roll of 11 misses (11 -1 = 10 --> lower then 15: MISS).
That's it people.
Why? Mostly because when somebody asks "What's thac0?" you can loudly proclaim, in your best nasal voice, "To Hit Armor Class Zero, of course."
As if that explanation alone is meaningful and makes sense. =P