Just for fun, I wanted to look into a system from a prior edition of D&D that has an infamous reputation: the "THAC0" system. THAC0 is an acronym, which stands for "To hit armor class 0," and was central to how combat worked in 2nd Edition D&D.
In a certain way, thac0 is not actually that complicated. But it's not straightforward either. Ultimately, the equivalent within 5th Edition is your attack modifier. The better this score is, the more likely you are to hit a target when you make an attack against it.
But while 5E's version (which is, as I understand it, basically unchanged from 3rd Edition) is pretty straightforward - higher number attack bonus is better for you, higher AC for the target is better for them - the way THAC0 works is far more convoluted.
First off, in 2nd Edition D&D, Armor Class was inverted - the lower your Armor Class was, the harder you were to hit. Essentially, it was like a rank - you generally want to be ranked first rather than second, so having an AC of 1 was better than having an AC of 2.
Your Thac0 score is essentially a target number: to hit a creature with a hypothetical AC of 0, this is the score you'll need to roll on your d20 in order to get a hit. So, let's say you have a Thac0 of 15. This means that if you're trying to hit a target with an AC of 0, you'll need to roll a 15 or higher on your die to succeed.
However, against a monster with a worse (remember: higher) AC, you'll be able to subtract their AC from your Thac0. So, say you have a Thac0 of 15, but you're fighting an enemy with an Armor Class of 6. Now, you only have to roll a 9 or higher on your d20 roll in order to hit them.
As you improve, your Thac0 will go down, making it easier to score a hit. But, tougher enemies will have lower ACs, even going into the negatives, which will effectively raise your Thac0 and thus require better rolls.
Now, you might be reading this and saying: hold up, isn't this just a weird and convoluted version of the system we use now?
And the answer is yes! Like in 2nd Edition, tougher monsters now tend to have better (in this case, higher) ACs but player characters also tend to have higher attack bonuses to overcome them.
If you have, say, a melee attack modifier of +5 and you're attacking a goblin with an AC of 13, then you're going to be hitting on a roll of 8 or higher. The equivalent situation could be like if you had a Thac0 of 15 and they have an AC of 7 - in both cases, an 8 or higher roll on the d20 will result in a hit.
But what I find interesting here is that the success or failure is presented as being on the player's side of the equation rather than the monster's. In 5th Edition, the difficulty of hitting a target kind of "ends up" at the target's AC, but it's our modifier that allows us to overcome a low roll of the die. In the Thac0 system, the "end" difficulty of hitting is instead on us, and it's the target's AC that is the modifier.
Functionally, you wind up at the same place, but it feels a little different. In fact, this actually reminds me a lot of other TTRPG systems, such as Blades in the Dark or Call of Cthulhu.
In the latter, your chance at succeeding or failing is entirely based on your character's statistics - you roll a number of d6s equal to your score in that particular skill, and hope that you get at least one 6 among them, which of course becomes more likely the better you are in that skill. That, interestingly, means that the GM in such games doesn't really have much input on whether you succeed or fail. Their role is entirely about adjudicating the consequences of failure, or interpreting what it means to succeed.
In Call of Cthulhu, your skills are on a scale of 1 to 100, and you roll percentile dice hoping to get underneath your score. Here, the GM has the opportunity to make a test more challenging, because the players will have a target roll for normal difficulty, but also one for hard and extremely hard difficulty (usually a factor of two or four smaller, so even if you have a 60 in, say, motor vehicles, you might still need to roll a 15 or lower in a particularly challenging car chase).
I definitely think that the current 5E system works better (though I'm also very eager to try MCDM's "no attack rolls" system). I think it's generally more intuitive when higher numbers are better, and it makes the formulation of AC a little more intuitive - like simply adding your Dexterity modifier to your AC, rather than potentially subtracting it.
I'm given to understand 1st Edition had an even more complicated system, upon which THAC0 was an improvement, but it does sort of surprise me that it took them so long to come up with the current system. D&D is turning 50 years old this year. 3rd Edition launched in 2000 (though 3.5 came three years later,) meaning that this change happened roughly halfway through D&D's existence from today's perspective. In other words, 26 years passed before anyone thought to make attacks and AC this intuitive.
I'm very excited for this year's (and next year's, given the release date of the Monster Manual) core rulebooks, and will likely start implementing the new systems in games I play (we'll see about converting my ongoing Ravnica campaign - I did let people swap to the new versions of their playable races when Monsters of the Multiverse came out). But I do think that there's a conservatism in the design philosophy (particularly in the effort to ensure that the books are backwards compatible) that makes me wonder if we're going to be burdened with things that we might be happier to shed in a true 6th Edition.
Granted, with the wealth of new fantasy adventure RPGs coming out (some of which truly seem to be trying something new with their mechanics) it might be ok if D&D itself remains similar in shape to how it has been.
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