Why THAC0 Rocks

Thondor · Mar 1, 2009

There has always been a rhetoric on these boards that discredits THAC0 (To Hit Armour Class 0) as being by far inferior to BAB (Base Attack Bonus). Posters state this as a matter of course, saying things like the adjustement to BAB (from THAC0) was so inherently logical that it is stunning that it didn't happen earlier.

I will attempt to explain here why THAC0 was created, how it works and why it is in fact simpler, speeds up play, and better then BAB.

Able to write a history of the evolution of THAC0 I am not. I know that in the 1e DMG both combat tables and THAC0 were used. And that BAB began in 3rd edition.

1. With THAC0 you know wether you hit on a tie or not. (This is the same with the saving throw tables in 1e/2e etc by the way.) There is no inherent logic that tells you wether you hit on a tie with BAB, wether you make a save on a tie, wether you beat the DC of a skill cheque on a tie. (Yes it says this on p.62, p.134, in the 3.5 PHB. However these are in isolated places and are not inherent to the definition. As far as I can tell, it does not actually say wether or not saving thows win on a tie.) With THAC0 the name itself says it all. If you have a THAC0 of 17, you must roll a 17 to hit a Armor Class of 0.

2. Ease of Use. A character who had a BAB of +5 would have a THAC0 of 15. Compare
A 3e character with a BAB of +5 tries to hit a enemy with a AC 15. Player rolls a 7, (ummm 7 plus 5 is 12) hey DM I got a 12. DM you miss. Next round, Player rolls a 13, (umm 13 plus 5 is uh 18), hey DM I got a 18. You hit. Next round, player rolls a 8 (umm, 8 plus 5 is 13), hey DM I got a 13. DM you miss.
A pre-3e character with a THAC0 of 15 tries to hit an enemy with a AC 5.
Player rolls a 7. Player says "7", DM says "you miss". (The DM knows that no one around the table has a THAC0 better then 14.) Next round, Player rolls a 13, says 13 DM says you hit (DM knows that everybody around the table but the wizard has a THAC0 of at least 18.) Next round, player rolls a 8, "I rolled an 8", DM is distracted "What's your THAC0?" Player "15", Dm thinks (15 -5 = 10) "You miss."

Let me explain that a little better in a different way and we'll get to another one of my points:
3. THAC0 means you do one calculation per different AC of opponent.
Same example THAC0 15 character trying to hit AC 15. This DM doesn't mind letting the players know the opponents AC.

Another player has attacked the same creature already. The DM informed them the AC was 5. Before rolling the player thinks to himself 'My THAC0 is 15 - 5 AC= 10, I need to roll a 10 or better to hit, sweet 50% chance!' (actually its a 55% chance) Informed that he's next, the player rolls a 7 "7! darn I miss." DM "OK next person". Next round, Player rolls a 13, "Yes a 13, I hit". Next round, Player rolls a 8, "8! I think I miss, 15-5 yeah I miss."

compared to:
DM says alright the AC is 15. the player has a BAB of +5.
First round: Player rolls a 7. 7+5 =12. "I got a 12, I miss."
Second round: Player rolls a 13. 13+5 =18. "I got a 18, I hit."
Third round: Player rolls a 8. 8+5 =13. "I got a 13, I misss."

With THAC0 you figure out what number you need to roll on the die once. With BAB you roll the dice, add your modifier, and compare it to the target number . . . every single attack.

I have found that THAC0 leads to much, much, quicker combat. For some reason people have an inherent fear of substaction, and have difficulty overcoming the initial conceptualization of THAC0. Which is really quite simple.

THAC0 - opponents AC = number you need to roll on the dice.

the 'complicated example' that many state of a negative AC is really simple: Say an AC -2, THAC0 15.
15 - - 2= 15 +2= 17. not difficult.

Chaos Disciple · Mar 1, 2009

Thondor said:
THAC0 - opponents AC = number you need to roll on the dice.

The advantage with the 3e version is that both the Target Number and BAB gain bonuses (this removes the confusion over the lower AC #= better AC)

But basiclly its the same formula, only reversed

Target number(Def) - BAB(Att) = number you need to roll on the dice.

Agamon · Mar 1, 2009

Personally, having played both ways for a number of years, I disagree, for two reasons:

1. Adding is inherently easier than subtracting.

2. Spreading the math from the one DM, who already does way more mental work than the players during the game, to the many players, is easier over the game night.

S'mon · Mar 1, 2009

"With THAC0 you figure out what number you need to roll on the die once. With BAB you roll the dice, add your modifier, and compare it to the target number . . . every single attack."

I agree - this does make THAC0 easier to use, where AC is known. You can do the same with ascending AC by deducting your attack bonus from the AC to get target number, but it's less intuitive.

BryonD · Mar 1, 2009

Thondor said:
compared to:
DM says alright the AC is 15. the player has a BAB of +5.
First round: Player rolls a 7. 7+5 =12. "I got a 12, I miss."
Second round: Player rolls a 13. 13+5 =18. "I got a 18, I miss."
Third round: Player rolls a 8. 8+5 =13. "I got a 13, I misss."

lol

So your evidence requires that the player roll a 13 one round and find out it is a miss and when he rolls an 8 the next round, for some reason he needs to "do the math" to find out if the 8 is better than the 13 or not.

Sorry, but the fundamental math is the same, just with BAB it is vastly more intuitive.

THAC0 isn't hard. But I could just as easily reverse your example and have the THAC0 player repeating his math every time while the BAB guy just remembers what he needed. It works the same. The only difference is whether or not one applies a bogus misleading example where the player does it correctly for one system and really stupidly for the other.

The math is the same. I have seen people get which number you subtract mixed up with THAC0. Not often, but it is easy to do by mistake.

BAB is easier because A+B = B+A.
THAC0 is less intuitive because A - B <> B - A

Lanefan · Mar 2, 2009

I've been running 1e for 25-odd years and still haven't figured out THAC0. To me, it just adds an unnecessary layer of complication - I don't care whether a swing hits AC 0, I care whether it hits the specific AC of the particular opponent at the time - all I want to know is whether the base roll, bonuses, penalties, fight level, and AC end up adding to 21 or more. THAC0 doesn't help me there, and instances of both the target's AC being 0 and the attacker's listed THAC0 rating being accurate for the situation happen rarely enough that I'm not going to waste the time checking.

Lanefan

Thondor · Mar 2, 2009

BryonD said:
lol

So your evidence requires that the player roll a 13 one round and find out it is a miss and when he rolls an 8 the next round, for some reason he needs to "do the math" to find out if the 8 is better than the 13 or not.

BAB is easier because A+B = B+A.
THAC0 is less intuitive because A - B <> B - A

Well that's my fault for copy an pasting and not editing. The middle one hits of course. So there is a reason for him to "do the math again". In my experience most players seem to do the math again with BAB anyway.
Seeing as you quoted me to reserve my error for posterity, I will go back and edit my first post in order that future readers arn't confused by my error.

In your example what's A and what's B? I'll be plain with you and admit I don't know what your getting at.

To me THAC0 is easier because the math is a constant, i.e. for one opponent and one attacker it is always the same.
eg: x (The only variable in either equation) is the number rolled on the dice.

Using THAC0: if x => THAC0 - AC it hits
Using BAB: if x + BAB => AC

So if a player wants to figuer out what they need to roll with BAB, they can. But they need to rearrange the equation in their head.
( x+BAB => AC
x + BAB - BAB => AC - BAB
x=> AC - BAB)
Which certainly isn't 'intuitive' and is actually exactly what THAC0 is. (S'mon basically said this above, far more eloquently then I have.)

The trick with THAC0 is figuring it out before the roll is made.

Umbran · Mar 2, 2009

It takes more verbiage to explain why THACO is simpler and easier than it takes to explain BAB to begin with. That says to me that THACO is not, in fact, actually simpler and easier.

YMMV, of course.

Thondor · Mar 2, 2009

Lanefan said:
I've been running 1e for 25-odd years and still haven't figured out THAC0. To me, it just adds an unnecessary layer of complication - I don't care whether a swing hits AC 0, I care whether it hits the specific AC of the particular opponent at the time - all I want to know is whether the base roll, bonuses, penalties, fight level, and AC end up adding to 21 or more. THAC0 doesn't help me there, and instances of both the target's AC being 0 and the attacker's listed THAC0 rating being accurate for the situation happen rarely enough that I'm not going to waste the time checking.

Lanefan

That's . . . unfortunate. You can always use those handy combat tables to speed things up!

Once again I'll have to admit that I don't understand what your getting at with your example, why would you want it all to add up to 21 or more? Ah I think I get you. AC 0 = AC 21 . . . which still leaves me confused. It's wonderful if the monsters AC is 0/21, because then THAC0 tells you everything you need to know -- you don't need to do any math. Just roll your THAC0 rating or better. In all other cases THAC0(including modifiers) - the AC (including modifiers) = what you need to roll to hit. untill that opponent is dead or you pick a new one -- which probably has the same AC.

Of course bonuses and penalties are rarer in AD&D then in 3e, especially modifiers that won't always effect your combat ability (eg spells, distance for ranged weapons). Still as long as you realize that every* number in 1e improves by going down, you should do fine. The lower the AC, THAC0 or Save the better. (even surprise is better the lower it is. surprised on a 1 on a d6, is better then surprised on a 3)
People seem to find this confusing because rolling higher is almost always better.

* a slight exageration. I believe Spell Resistance is better the higher the % is. I can't think of any other subsystem (psionics? maybe) that does though.