The second edition of Dungeons & Dragons had a bizarre rule known as THAC0, which was used to determine if attacks hit or missed during combat. The rule was needlessly complicated and it acted as a barrier to entry for new players.
The rules in the current edition of D&D are easy to explain to new players. This is helped by the fact that a lot of the math has been taken out of the game. In the older editions of D&D and in games like Pathfinder, the DM and the players were constantly adding or subtracting numbers. The current version of D&D has toned all of that down, due to most scores being worked out at the start and gradually upgrading over time.
The second edition of D&D (known as Advanced Dungeons & Dragons) is still beloved by a lot of people, as it’s the one that was used in games like Baldur’s Gate. The people who weren’t familiar with AD&D often struggled during the first few hours of Baldur’s Gate or its sequel, as putting on armor actually reduced the player’s armor class score. The reason for this was due to how the combat system was tied to an unintuitive rule.
Combat in the current edition of D&D involves working out two numbers and a dice roll. The first number to work out is Dungeons & Dragons’ armor class (or AC) score. AC is 10, plus the bonus granted from the character’s armor, plus their Dexterity modifier, plus any additional modifiers (such as carrying a shield). The second number is the attack bonus of the character, which is their proficiency bonus, their Strength bonus for melee attacks or Dexterity bonus for ranged attacks, and any miscellaneous modifiers. If a character wants to hit someone, then they need to roll the same number or higher than the AC of their target.
In order to explain combat, let’s imagine a goblin with an AC of 12 is being attacked by a Dungeons & Dragons Fighter with an attack bonus of +4. The Fighter’s player would need to roll an 8 or higher on a d20 in order to hit the AC of the goblin. If they roll a 1-7, then the attack misses.
The combat system is easy to follow, as most of the numbers explained are worked out early in the game by the player and they only increase in small increments over the course of the campaign. By the same token, the AC/attack bonus numbers are laid out for the DM in the Monster Manual, so they only have to work them out if they’re making a homebrew D&D campaign.
In AD&D, a character who isn’t wearing armor is considered to have an AC of 10. If they put on armor, then that number goes down. A character who puts on field plate mail armor has their AC reduced to 2. In order to work out the number needed to hit that character, the player would need to determine their THAC0 score.
THAC0 is an acronym for To Hit Armor Class zero. A character’s THAC0 score determines the number they would need to hit an AC score of zero. If a character’s THAC0 was ten, then that meant they needed to roll a ten to hit an AC of zero, a nine to hit an AC of 1, or an eleven to hit an AC of -1. Rolling above the THAC0 score also countered as a hit. Negative numbers were part of the equation, and they became more important as the campaign went on. Characters in Baldur’s Gate 2 could easily hit the low minuses for AC, once they piled on magic armor, rings, and Dexterity modifiers.
It was common for DMs and players to fill out a THAC0 chart and keep it to hand at all times, as it was easier than having to keep track of the numbers. It was a pain whenever a character gained a temporary THAC0 boost from an enchantment, as it temporarily threw the numbers out of whack. The third edition of D&D has its detractors, but there is no denying that replacing THAC0 with a simpler combat system was one of the best things it brought to Dungeons & Dragons.