THAC0 has been discussed elsewhere but I want to give it a look here for two reasons. First I find it interesting enough to be worth understanding. Second it would seem that the acronym has become the butt of a joke being told by people who are old enough to know better. Let’s begin with the Eric J. Holmes boxed set because that’s where I began.

What we now call “Holmes Basic” or “Basic” Dungeons and Dragons had only three levels of play. The player’s were given a chart to use (below) to determine whether they could hit various Armor Classes. A similar table was also provided for the monsters. The Holmes edition refers players to Advanced Dungeons and Dragons for levels beyond three but in reality two other versions of Dungeons and Dragons B/X or Moldvay and BECMI or Mentzer would be published.

DIE ROLL FOR CHARACTER TO SCORE A HIT, BY OPPONENT’S ARMOR CLASS¹

Armor Class	9	8	7	6	5	4	3	2
	No Armor	Shield	Leather Armor	Leather & Shield	Chain Mail	Chain Mail and Shield	Plate Mail	Plate and Shield
Normal Man	11	12	13	14	15	16	17	18
1st-3rd Level Character	10	11	12	13	14	15	16	17

Both AD&D and the various D&D boxed sets would go on to have charts similar to the above describing what number a character of a given class needed to roll to hit a given Armor Class. There is some difference between the AD&D and D&D charts since both games offered different classes but the charts were similar to the extent that they both had different to hit numbers by character class with Fighter types progressing fastest and the Wizard types progressing the slowest. Clerics types and Thief types were in between.

The D&D character sheet made room for a hit roll chart. The player could roll a twenty-sided die, add their relevant modifiers and simply tell the Dungeon Master, “I hit AC 5.” To the DM that was a signal that the player had hit any AC 5 or higher (higher AC being worse) and missed any AC lower than 5.

AD&D moved the to hit charts into the Dungeon Master’s Guide. The character sheet did not make room for a hit roll chart. As a consequence AD&D players typically rolled a twenty-sided die, added their relevant modifiers and told the DM what their adjusted to hit roll was. The DM then looked up the result on the proper chart and let the player know if they had hit.

An observation could have been made by anyone analyzing the charts of the above games and certainly must have been made or known to the developers of Second Edition Advanced Dungeons and Dragons. That observation is that AC 0 is the point around which to hit rolls pivoted. Which is to say that starting at 0 the number needed to hit AC was one lower each time AC went up and one higher each time AC went down. Put another way if a player needed a 15 to hit an AC of 0 then a 14 was needed to hit AC 1 and a 13 to hit AC 2. Conversely a 16 was needed to hit an AC -1 and a 17 to hit AC -2.

2E changed the rate at which characters improved at hitting AC and made advancement more regular while keeping the same philosophy of Fighter types improving fastest with Wizards types bringing up the rear with Cleric and Thief (now Rogue) types in the middle. The chart to explain this was moved to the Player’s Handbook and greatly simplified from several pages of charts in AD&D to a single number the player could note on their character sheet; To Hit Armor Class Zero or THAC0.

THAC0 allowed 2E players to return to rolling a twenty-sided die, adding their relevant modifiers and telling the Dungeon Master what AC they were able to hit. AC was arrived at by subtracting the modified to hit roll from THAC0. For example a player with a THAC0 of 16 and a Strength of 17 (+1 to hit) would roll a d20 add add 1 for their Strength and subtract that number from their THAC0. If the player rolled a 13 the math would look like this (16 – (13+1) = 2) and the player would announce they had hit AC 2.

If you haven’t played these versions of D&D all of the above may seem arcane but if you’ve made it this far there is a moral to the story. THAC0 reduced what were several charts in earlier editions of D&D to a single number and placed responsibility for that number back in the hands of the players.

Conclusions

I’ve mentioned elsewhere that I started playing D&D in 1985. At the time I was 11 and lacked the vocabulary and maturity to really understand the rules. When 2E was released the vocabulary/maturity gap had dissappeared; I was 15. From my perspective THAC0 was a vast improvement over flipping though the pages of the DMG to find a chart to tell the players if they had hit or not. I’m sure the internet is full of slightly older players that didn’t like THAC0 very much.

At some point later in D&D‘s evolution someone decided that ascending AC would simplify to hit rolls even further. I can’t disagree but I do wonder if something has been lost with ascending AC.

In the games I mention above the Fighter is always the best with weapons. Other non-fighter types lag. That lag may reflect the Fighter’s focus on weapons or the it may reflect the fact that the other classes have special abilities and magic spells. It may also represent some other factor or factors I haven’t considered. In any case a question must be asked. Should a character whose life is dedicated to swinging weapons be equalled in ability to hit by some other character whose focus is otherwise?

That may be a question for each of us to answer individually but the undeniable truth of more recent D&D games is that a statistically similar Fighter and Wizard (proficiency and ability score) given the same weapon have the same chance to hit and therefore damage a foe. Should a character that can throw a Fireball be as effective with a Longsword as a character who can not?

We can conclude from the rules of recent D&D games that a character that can throw Fireballs should indeed have an equal ability to hit with a Longsword. In return Fighter types gain exceptional abilities—and in some cases spells—of their own.

These new features are nifty but I can’t help feeling like something unique and beautiful has been lost.

Foot Notes

1. Gary Gygax and Dave Arneson, DIE ROLL FOR CHARACTER TO SCORE A HIT, BY OPPONENT’S ARMOR CLASS: DUNGEONS & DRAGONS (Lake Geneva: TSR Games, 1978).