The Most Confusing Rule In Dungeons & Dragons

Now that Dungeons & Dragons is over 40 years old and well into its fifth edition, the roleplaying classic has become a pretty streamlined affair. While the game still requires more of a commitment from players than a simple board game does, the ruleset has been refined to the point that anyone with a basic understanding of roleplaying mechanics can join a campaign and have a good time. Of course, this wasn't always the case. Wizards of the Coast didn't implement the core d20 system with which most modern Dungeons & Dragons players are familiar until the 3rd edition was released in 2000, over 25 years after the game first came out. During those years, the game drew from various sources to put together combat mechanics, including old tabletop wargames. The resulting rules were sometimes disjointed and often confusing, which kept the roleplaying game a niche product at best.

While those archaic rules are more or less a thing of the past, they do pop up in classic video games based on Dungeons & Dragons, such as Baldur's Gate and Planescape Torment. Whenever gamers today have to face an older version of Dungeons & Dragons, there is one famously confusing element that players need to wrap their minds around — the clunky and confusingly named THAC0.

Dungeons & Dragons started as a small project for creator Gary Gygax. The original version featured only a few of the different classes and alignments that would become a complex component of later editions. The original combat system was rudimentary, with only a single page detailing combat rules, according to Dungeons & Dragons. By the time the second edition of Advanced Dungeons & Dragons was released in 1987, the game had grown massively in terms of the amount of content and the importance of combat. In a misguided attempt to codify combat rules across the game's different settings and simplify the game for players new and old, the second edition introduced a new mechanic to determine a character's attack score. That rule was known as To Hit Attack Class 0, or THAC0.

Essentially, what THAC0 indicated was the die roll a player needed to hit a target if its armor class was zero, which was the best armor rating possible without magical aid (via Retro RPG Library). As such, a level-one character would have a THAC0 of 20, meaning it would be next to impossible to hit a well-armored target. A level 10 character, however, would only need to roll a ten or above to hit. It sounds simple until you take into account two factors. First, almost nothing in the game has an armor class of zero, which means that attack rolls required constant subtraction and addition. Second, it meant that better armor, especially magically improved armor, lowered a player's armor class. The best armor in Dungeons & Dragons, which was rarely encountered in standard sessions but prevalent in games like Baldur's Gate 2, would send player's armor class far into negative numbers. The system was unintuitive and confusing, ultimately creating a significant barrier for new players.

While the THAC0 system has demonstrated itself to be counterintuitive, it was intended to simplify things. On Reddit, one user posited that the system had its origins in naval wargames that creator Gary Gygax was familiar with. In those games, Gradenko_2000 explains, "The term Armor Class refers to ships: how thick, and how well-covered the ship was in armor plates. An AC of 1 was very good: it meant first-class armor. AC 2 meant second-class, and so on."

The critical difference between those systems and the more dynamic game Dungeons & Dragons became during the '90s was that a warship's armor never changes. Likewise, Gygax didn't anticipate a game in which spells, magic items, and potions were continually modifying characters' offensive and defensive values. Instead, Gygax assumed that players would be able to draft a simple chart at the beginning of a session, based on their character's current level and equipment, to refer to throughout the game. If THAC0 had worked the way Gygax intended, it would have nearly cut out the need to perform any math mid-session. Instead, the constant change of values based on the variables in play made determining the results of an attack less straightforward than it should have been.

The third edition of Dungeons & Dragons, which was the first produced by Wizards of the Coast, eliminated THAC0 and replaced it with the far more robust d20 system. A Reddit post by GodotIsWaiting4U notes that the d20 system added more math that players need to perform to calculate bonuses and modifiers — but at least all the numbers moved in the same direction. These simplified rules have helped make the game appealing to a new generation of roleplayers and may have helped the game even land a new Dungeons & Dragons movie.