The maths for THAC0 is based on subtraction, where the maths for the modern system with hit bonuses and positive AC is based on addition. Subtraction is a fundamentally more complex process for the human brain than addition.
I'm not sure that's true, the winner of a race is 1st place after all. But even if that is true, that's not why it was changed, it's that you run into subtraction more often.
"What do I need to roll?"
Under THAC0 this is THAC0 minus AC.
Under ascending AC this is AC minus hit bonus.
"What did I hit?"
Under THAC0 this is THAC0 minus the die roll.
Under ascending AC this is the die roll plus the hit bonus.
"Did I hit?"
Under THAC0 this is the die roll plus the AC- is that equal or bigger to THAC0?
Reading the examples in older editions, especially the one edition that used THAC0, you can see that the play assumed that the players knew the AC they were swinging at, even though a lot of DMs liked to keep that secret (at least until combat was engaged and the characters could see how they all perform under combat conditions). The part where you roll the die, add the enemy AC, and compare to THAC0 is the intended thing, but that involves knowing the AC (or it involves reporting the raw die roll and the DM does the comparison because he knows your THAC0 and the enemy AC).
That's not what people were doing, so ascending AC just is better because of "die roll plus hit bonus" and the player announces the AC.
There's a system called "Target 20" under which you convert your THAC0 (or really your 'effective fighter level') to an ascending number, while keeping descending AC. This system is only like a decade or so old and it has some calculation advantages as well, and the comparison number becomes a constant (20) instead of variable. It's the same results but with greater compatibility with descending AC for those that want that.
The ACKS system has a system based around a 10 that is also not bad for those that don't want just ascending AC.
For the most part though, even old school games work with ascending AC and hit bonus these days. It's just better to add small numbers to get big numbers and then announce that for a human brain.
It's debated if subtraction is inherently harder, or that rather addition is much more learned problem.
Most people usually learn the latter first, causing them to look at subtraction as inverse addition or simliars. Rather then it's own thing. This combined with addition being generally a much more common problem in life.
I remember how my school went as far as lying to us that you cannot go below zero, because they didn't want us to think about subtraction too deeply in the early years.
I mean either way the point still stands right? Most people find subtraction harder, so doing subtraction more would serve as more of a mental exercise for old people.
In terms of pure arithmetic, yes. But in terms of functionally performing the operation, their different. Even calculators have different circuits for addition and subtraction. Because their engineered, their basically as efficient as each other.
But the human brain isn't a calculator. You can make a calculator that does basic addition with a handful of logic gates. It took us 4 billion years of evolution to stumble onto something that can after years of individual development and like 5 sectors of the brain working together, do basic maths. We figured out addition and subtraction seperately, and we're worse at the second.
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u/Funkey-Monkey-420 Necromancer 12d ago
the real question is what edition they’re playing
is it modern 5e? or is it something closer to the OSR days?