THAC0, armor class, saving throws, skill, and other applications of the d20

More than a few times, when I’ve told people that I play an older version of D&D, I’ve gotten a slightly horrified look and the question, “Is that the one with THAC0?” What’s so awful about THAC0? I ask, but the answers are vague. “It doesn’t make any sense! It’s bizarre!”

I think that most of the time, “the one with THAC0” means the second edition of AD&D, but pretty much every D&D before third edition had THAC0. It just means that your character has a certain target number to hit something with armor class zero. To hit armor class 0.

So, you roll a twenty sided die, add your target’s armor class, and see whether your result is the target or better. This is basically how every single roll works in third edition, so why is it weird?

  d20 + modifier >= target

I think people get confused because it’s tied to descending armor class, where it’s better to have a low armor class. Of course it is! If your AC is a bonus to your enemy’s to-hit roll, you want it to be low. The third edition system is mathematically identical. It just swaps the modifier and target. The enemy now determines the target, not the modifier, because the target is the enemy’s armor class. The character’s modifier is now (basically) constant based on his level.

So stop complaining that THAC0 is confusing!

One thing I like about THAC0 is that it is a little different than what people are used to. I think later editions of D&D try to boil everything down to one universal mechanic, which makes it harder to simply drop one optional bolt-on (like the seafaring rules from Cook’s expert rules) or add other optional rules (like psionics). If there’s a universal mechanic, everything needs to fit into them. If everything has its own simple, self-contained set of rules, you can monkey around without breaking the whole game.

I’m still thinking about non-combat challanges in this context. (This is where a “skill system” would often come into play, but I don’t like the implications of enumerating all a character’s skills.) A common mechanic is to try to roll at or under the relevant attribute. So, if you’ve got a 14 Charisma and you’re trying to intimidate the town guard, you’ve got to roll under a 14. A natural 20 is the worst you can do, so it’s a critical failure. A natural 1 is a great success. On a tie, the character with the higher attribute wins.

Building from that, some systems say that instead of 1 being perfect, you shoot for your attribute’s value. That way, in a Strength contest between two characters, the winner is the one who rolls highest without exceeding his or her score wins the contest. Attribute scores don’t need to be revealed or compared. Zak S. wrote about this, and points out the obvious (if silly) problem: it’s not very exciting for everyone else at the table to see the die roll and stop on 13, even if that is the critical success value. Everybody wants to make a big noise at a 1 or a 20.

I’ve been thinking about this a lot lately as I prepare to switch my 4E campaign to a hacked B/X. I’m thinking about extending my list of saving throws to add a few more categories and just using that. I’m also tempted to just say “we’re gonna use FUDGE dice” and bring in FAE-style Approaches, because those seem pretty awesome.

As usual, what I really need is more table time to just test a bunch of bad ideas and see which is the least worst.

This post has been sort of rambling and pointless. Please allow me to pay the Joesky tax:

The high priests of Boccob are granted knowledge of secrets and portents, but often at great price. Some of these powers (initially for 4E) are granted to the highest orders while they undertake holy quests:

Boundless Knowledge. Can learn any fact through a turn of meditation. The cost:

  • widely known mundane fact: 1 awesome point
  • secret (secret keeper present): 10 awesome points
  • secret (secret keeper not present): takes 8 hours, 10 temporary wisdom, recover 1/day
  • arcane secret: 1 permanent wisdom
  • mystery of the universe: takes 1d4 days, costs 1d4 permanent wisdom

Can’t recover awesome points if not used once per day. Begin losing 2 temporary Wisdom per week after a week without learning an arcane secret.

Subtle Stars. Every night, the PC can consult the stars and learn two facts and one lie. Failure to consult the stars once a day leads to a -2 cumulative penalty to Will defense.

Curiosity. Every time the priest asks a question that goes unanswered (even in soliloquy), he must roll a d20. If it is more than his Wisdom + 2, he gets the answer and loses a point of Wisdom.

Written on August 31, 2013
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