In Defense of THAC0

The year is 1981. Your trusty human fighter swings an axe at the goblin…aaaand it’s time to break out the attack matrix to find out if you hit. A lot of people keep asking why we’re not using something like the combat charts below, or THAC0 in The Secret of Weepstone, and let me tell you exactly why we’re keeping it simple.

In reality, after a while of playing any roleplaying system, whatever arcane methodology the designers use to determine something like success in combat usually does become easier to do on the fly. That said, there’s been many, many more ways to calculate combat in Dungeons & Dragons in addition to the charts above, so there’s no one “true” answer or solution to it, or for a game paying homage to it. So… let’s talk about the one that seems to be the most beloved and hated in equal measure, THAC0.

If you’re somehow this deep into a blog dedicated to retro RPG combat resolution mechanics and don't know what THAC0 stands for, hats off to you, but it stands for ‘To Hit Armor Class 0’.

What’s the deal with THAC0?

Like the examples above, many methods over the years were used to figure out how to represent the difficulty of a given monster or character succeeding in an attack against a foe. Different accounts are given of how they landed on THAC0 specifically, who called it that first, and the very first time it was officially used as the method. What’s certain is that the mechanic appeared in an un-named form in AD&D 1st Edition (1977), only to become the official method for AD&D 2nd edition (1989).

Another notable place that people became aware of THAC0 was the original Baldur's Gate game series from Black Isle. As it was made in the 90’s, it used the current ruleset, which was 2nd Edition AD&D, in all its THAC0-laden glory. That being said, with a computer doing all the calculations, the player didn't need to follow what was happening under the hood. The differences are more obvious when playing at the table in person. Ascending Armor Class is what modern (players since the release of D&D 3rd edition in 1999) would recognize, where you’re adding armor and any other modifiers together to find a target you’re attempting to roll equal to or above. Higher Armor Class is always good, thus it’s called Ascending, whereas in THAC0, adding armor lowers your Armor Class as you get more protected, leading to it being known as Descending armor class.

One of the common jokes among players of older editions is to explain THAC0 to someone familiar with the more modern armor classes. The feature that always stands out as the strangest is that it is possible to lower your armor class below zero. You might be asking, "Then why use THAC0, a system that, in theory, stops at zero?" Well, it doesn't, but how that works is pretty funny. You always subtract a target’s armor class from your THAC0 to determine the target to hit (Example: A character with THAC0 of 18 targeting a foe with AC 8 only needs to roll a 10 to hit.) but if your target has a negative Armor Class, you need to subtract the negative number from your THAC0, which of course means you have to add it.

Where’s the defense?! I said I was going to defend THAC0, so here we go. It’s wonky, and to modern rules