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THAC0 5e

Added 2020-11-11 13:29:05 +0000 UTC

There's an old D&D system that you've probably heard of.

It's called THAC0, which stands for "To Hit Armor Class Zero."

D&D 1st Edition's "To Hit" charts were a precursor to THAC0.</figure>

Whether you pronounce it thay-co or thack-oh, if you weren't contemporaries, you probably heard about it in a less than favorable light.

The reason it became the butt of jokes was a combination of factors:

It was a relatively simple concept explained in a convoluted manner. When THAC0 was moved into the AD&D Player's Handbook and officially termed, the explanation was confusing enough that you could fairly read it backwards.
On a more mechanical level, the "roll low" system was counterintuitive. Western society generally conditions us to believe that "more is better" and it's mentally jarring to break from that trend.
Finally, addition is also the most basic form of arithmetic, and works a little bit faster than the subtraction you needed to calculate THAC0.

But, the point of THAC0 wasn't to sit around calculating THAC0. It was a reference table to make things faster for DMs.

In this respect, it was superior to our current system. It speeds up combat by taking attack math off the table. The DM can say "roll a d20" and when the die falls, they know whether or not it's a hit. There's no arithmetic between the roll and the resolution.

When we flipped from "low is good"/subtraction to "high is good"/addition, we lost a handy reference material in the process. The good news is that this system is still relatively easy to implement! But, it's going to require some subtraction.

What you do is subtract the attacker's attack bonus from the target's armor class (AC). That gives you the number you need to roll to hit. If you're not comfortable doing that on the fly, here's a tidy little chart you can tape to your DM screen.

5e Rules "To Hit" table showing d20 roll needed based on Attack Bonus and Target AC</figure>

By using this chart, you can evaporate the math break between the roll and the excitement. When the player declares their intention to make an attack, tell them "roll a d20, X to hit." When the die stops rolling, everyone at the table will know whether or not it's a success! This is the most dramatic moment in play! Don't suck excitement out of the moment by injecting a math break.