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The truth about THAC0

THAC0 is always raised as the big thing that made oD&D and AD&D totally unplayable. (for D20, the counterpart tends to be attacks of opportunity )

So lets deconstruct the math and see how it works.

I'll use AD&D to refer to the pre-D20 editions (as they all works the same, hit roll wise) and 3.x to refer to the D20 editions.


Finding out what you need to roll equal to, or better:

3.x: Deduct Base attack bonus from armour class. Result is number you roll equal to or better to hit.

AD&D: Deduct armour class from THAC0. Result is number you roll equal to or better to hit.

(Exactly the same)


Finding out what armour class you hit with the number you just rolled:

3.x: Add the rolled, modified number to your base attack bonus

AD&D: Deduct the rolled, modified number, from your THAC0.

(the same, except one is a deduction instead of a plus)


Basic to hit roll mechanism:

3.x: Roll 1D20. Add base attack bonus. Equal or better than armour class is a hit.

AD&D: Roll 1D20. Add targets armour class. Equal or better than THAC0 is a hit

(exactly the same)


Looking up numbers on a chart ? Both versions use two single numbers to determine attack success, both of which would be written on your character sheet, and unchanging till next level (Thac0 / BAB and armour class)



Then there's armour class.

3.x AC: Number shows how hard it is to hit you. Thus, a high number is good.

AD&D AC: Number shows how easy it is to hit you. Thus, a low number is good.



Saving throws are the other thing, but I'll leave that for another day.

Mind, this isnt an edition war attempt but a basic deconstruction showing that THAC0 works almost exactly identical to the D20 system.
 

arwink

Clockwork Golem
Mechanically they're much the same.

In the mind of a new player, and many experience players, 3e's attack system is much more intuitive than THAC0. While I had players who couldn't tell me what AC they hit after a year of play in 2e, they picked up the 3e system in a matter of hours.
 

Nathal

Registered User
They're weren't too many monsters in AD&D with ACs better than -7, right? A character with a THACo of zero would need to roll 7 or higher on a D20 to harm that creature. In 3E those numbers pale in comparison. The new rules seem to involve much bigger numbers than what earlier editions dealt with, but maybe I'm not remembering it right.
 
The problem IMO wasn't so much THAC0 itself (though that was occasionally challenging as arwink describes), but that you had to have a table to look up THAC0 in the first place (let's see, I'm a fighter level 6, so I use this line, and ...). And a table for saves. And a table for monster attacks.

Now at least I don't need the tables.
 

Crothian

Villager
Olgar Shiverstone said:
The problem IMO wasn't so much THAC0 itself (though that was occasionally challenging as arwink describes), but that you had to have a table to look up THAC0 in the first place (let's see, I'm a fighter level 6, so I use this line, and ...). And a table for saves. And a table for monster attacks.

Now at least I don't need the tables.
The THAC0 tables were just a forumal written out in table format. It was easy to memorize and figure out what the to hit number was. Saves most people wrote down and that was also easy. THe nice thing was the numbers wqere easy, no getting bonuses from a half dozen different sources like in d20.
 

fanboy2000

Villager
Crothian said:
no getting bonuses from a half dozen different sources like in d20.
I wonder if that's why they went to the BAB system, they wanted a way to take diffrent bonuses into account.
 

Viktyr Gehrig

Villager
Crothian said:
The THAC0 tables were just a forumal written out in table format. It was easy to memorize and figure out what the to hit number was.
Indeed.

2e:
Fighters reduce THAC0 by 1 per level.
Clerics reduce THAC0 by 3 per 4 levels.
Thieves reduce THAC0 by 1 per 2 levels.
Mages reduce THAC0 by 1 per 3 levels.

3e:
Fighters increase BAB by 1 per level.
Clerics and Rogues increase BAB by 3 per 4 levels.
Wizards increase BAB by 1 per 2 levels.

THAC0 was simple. It's saving throws that were a mess.
 

Saeviomagy

Villager
I'm pretty sure that thaco was not all that there was to it.

For instance: How many 19s and 20s were there on the end of the attack table? How many 2's at the other end?

I'm fairly certain that the table had something like 3 19s before 20... Can anyone tell me if I'm wrong?
 

Viktyr Gehrig

Villager
Saeviomagy said:
For instance: How many 19s and 20s were there on the end of the attack table? How many 2's at the other end?

I'm fairly certain that the table had something like 3 19s before 20... Can anyone tell me if I'm wrong?
Before it was revised (I think in the second printing of the PHB, but I may be wrong), Fighters got 2 THAC0 per 2 levels, instead of 1 per level.
 

Crothian

Villager
Saeviomagy said:
I'm pretty sure that thaco was not all that there was to it.

For instance: How many 19s and 20s were there on the end of the attack table? How many 2's at the other end?

I'm fairly certain that the table had something like 3 19s before 20... Can anyone tell me if I'm wrong?
It does have 6 20's in it, yes. but not a lot of twos, in fact the numbers go negative. So, the forumalu just has the 20's rep[eated the six times and then goes on from there.

First level fighter nmeeds 20 to hit AC 0, and a 20 to hit AC -5, a 25 to hit -10
 

Staffan

Explorer
Korimyr the Rat said:
Indeed.

2e:
Fighters reduce THAC0 by 1 per level.
Clerics reduce THAC0 by 3 per 4 levels.
Thieves reduce THAC0 by 1 per 2 levels.
Mages reduce THAC0 by 1 per 3 levels.
Actually, priests reduced THAC0 by 2 per 3 levels.
 

whydirt

Villager
THAC0 wasn't that bad, but I think the BAB system is A. more intuitive to new players B. more consistent with the rest of 3rd Edition's rules.

I think the part that was the most confusing wasn't THAC0 itself, but that Armor Class started at 10 and worked its way down to below 0. Now Armor Class is essentially just another Difficulty Class with a specific name kept from older editions.
 

MerricB

Eternal Optimist
There were two issues:

The first was mathematical. People add much faster and more accurately than they subtract. When you have subtraction of negative numbers, things get hairy for the less mathematically competent amongst us.

For a fighter with a THAC0 of 14 to strike a monster of AC -2, the fighter had to roll a 16 or higher (14 - (-2) = 16).

I had someone in my old group who I had to explain how THAC0 worked every single session. This was a problem.

The second was due to inconsistency between whether rolling high was good (saves and attack rolls) or whether rolling low was good (attribute and proficiency checks).

Jonathan Tweet notes that the attribute/proficiency system (rolling low) also worked very badly for opposed checks... it ended up being who rolled higher without rolling higher than their skill! The unified d20+modifiers check of 3e fixed that problem very nicely.

Cheers!
 
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Jdvn1

Villager
/me seconds the inconsistency arguement.

The d20 system is easier because the entire system is "meet or beat this number." You always know you want to roll high. Mechanically, the same, logically simpler.
 
AD&D1 and classic D&D uses the "repeating 20".

AD&D 2nd edition didn't. The chart was even, throughout the advancement.


Also, as I illustrated, you dont need to subtract anything. Just add the armour class of the target to your roll, and score equal or higher than THAC0. Same mechanic as 3.x ;)


Saving throws were rather arbitrary but they still are. Of course you find them in the same way. Look on a chart, write down the number.
 

Caliban

Rules Monkey
weasel fierce said:
AD&D1 and classic D&D uses the "repeating 20".

AD&D 2nd edition didn't. The chart was even, throughout the advancement.


Also, as I illustrated, you dont need to subtract anything. Just add the armour class of the target to your roll, and score equal or higher than THAC0. Same mechanic as 3.x ;)
Sure. It's not like they made a secret of that. It was talked about by the designers when 3.0 was first released.

Saving throws were rather arbitrary but they still are. Of course you find them in the same way. Look on a chart, write down the number.
Saving throws aren't arbitrary, they follow a simple formula: 1/2 your level +2 for "good" saves or 1/3 your level (round down) for "bad" saves.

I haven't looked on a chart for saves (unless I'm feeling lazy) for quite awhile.
 

ThirdWizard

Villager
weasel fierce said:
Also, as I illustrated, you dont need to subtract anything. Just add the armour class of the target to your roll, and score equal or higher than THAC0. Same mechanic as 3.x ;)
I think the PCs knowing the AC of their enemy is a pretty big assumption. That's definately not any information I ever handed out.
 

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