THAC0 (“To Hit Armor Class Zero”) isn’t difficult to understand, despite what the rumors say.
This is *especially* true if you are a player (that is, not the game’s referee).
It is only marginally more complex than what is known as “ascending armor class”, where the player rolls a twenty-sided die, adds any bonuses or penalties, and tells the referee the result.
With THAC0, as a player, one rolls a twenty-sided die, adds any bonuses or penalties, subtracts the total from 19, and then tells the referee the result.
The player’s part is finished.
The referee compares the number told him by the player to the target’s armor class. If it is the same or lower than the target’s armor class, then the attack is successful, otherwise, the player’s character missed the target.
For example, the player rolls an attack, and gets a result of 16. He subtracts 16 from 19, a difference of 3. He tells the referee, “I got a three”, or, perhaps, “I hit an AC of three.” The referee checks the target’s armor class, which is a 5, and replies, “You hit the monster.” Or, the armor class is a 2, in which case the referee says, “You miss.”
1. To reiterate, if the player’s character has a bonus or penalty, then said bonus or penalty is applied to the die roll before subtracting from 19.
For example, if the player’s character has a strength score of 16, which grants a two point bonus to melee (hand-to-hand) attacks, then the player will add two to the roll, and then subtract *that* number from 19, and tells the referee the final result.
If the player’s character has a strength score of 7, then the character has a one point penalty. He must subtract one point from the die roll prior to subtracting the result from 19.
2. Depending on a character’s adventuring profession, he will improve his ability to hit armor class zero. Fighters will improve THAC0 by one point when they reach the fourth level of experience (this is in the Cook-Marsh Expert Rules). A fourth-level fighter will subtract a d20 roll from 18 instead of 19 (after applying bonuses or penalties).
Congratulations!
You have completed this course in Old School Table Top Roleplaying, earning 3 credits toward your degree as a Master Adventurer, and the coveted “THAC0 Genius” badge. Go forth to victory!
Greymerry Faire 