What does THAC0 stand for in Dungeons & Dragons?

To Hit Armor Class 0

If you’re into the ins and outs of old school Dungeons & Dragons (DnD), you might have heard of THAC0. It’s an acronym that stands for To Hit Armor Class 0, a rule from previous versions of the game.

Notably in AD&D Second Edition, a character’s armor class (AC) would decrease as they put on better armor. Yep, it’s as confusing as it sounds. Both player characters and non-player characters had THAC0 scores, which showed the number they needed to roll on a d20 to hit an opponent with an AC of 0. The higher the character’s level, the lower their THAC0 score, and the easier it was to hit opponents.

To see if they hit an opponent, a player would roll a d20, check their THAC0 score, subtract the opponent’s AC from their THAC0 score, and compare the result of their roll to the adjusted THAC0 score. If the roll’s result was the same as or more than the adjusted THAC0 score, the player hit the opponent. Or, players could skip the math and look at a detailed attack matrix that showed the number they needed to hit.

With the arrival of DnD Third Edition, THAC0 was tossed out. The math in DnD combat is now a lot simpler. Most players won’t ever need to know what THAC0 means or how it was used. That being said, some old-school players like to show off their knowledge of THAC0 to emphasize their long-time fandom.

Example for using ‘THAC0’ in a conversation

Hey, I found this old Dungeons & Dragons book with some crazy rules!

Oh really? Like what?

Well, there’s this thing called THAC0. It stands for To Hit Armor Class 0.

What does that even mean?

Basically, it’s a way to determine if you hit an opponent in the game.

Sounds complicated. How does it work?

You roll a dice, subtract the opponent’s armor class, and if the result is equal to or higher than your THAC0 score, you hit them.

Wow, that sounds like a lot of math!

Yeah, it was pretty confusing. But thankfully