Descending AC makes perfect sense and the only flaw is how bad people are at basic math.

Yes, but designing a game that favors how people already behave is designing a game for humans. Adding mechanics that act like

Norman doors (YouTube link) is not good, and it's pretty indefensible from a design standpoint.

The problem with THAC0 and descending AC is exactly that the math is error prone. It doesn't have to be

*difficult* math for it to be error prone design. It's error prone because addition is associative and subtraction is not. That's it. Ascending armor class eliminates as much of this problem as you can, while descending armor class means you get to deal with it every time. It's even worse when your values are often close together. If you have three values, 15, 16, and +1 and you need to subtract two of them from the other, you notice very quickly that you're going to end up with values around 0 no matter how you do it.

16 - 15 - 1 = 0

16 - 15 + 1 = 2

15 - 16 - 1 = -2

15 - 16 + 1 = 0

It's very difficult to tell that you've made a math error because all those results are within the range you'd expect to hit. The fact that you can do the math wrong and potentially still get the right answer is

*even worse*. The very fact that

*you can't tell* which of those formulas above is correct until I tell you which one is the die roll is the problem.

If I give you three numbers 5, 7, and 2 and tell you to add them up, you don't have any problem at all doing it and getting the right answer. It doesn't matter what order you do them in. It doesn't matter that you don't know which roll was the d20 roll and which one is the attack bonus. That information is not relevant to the calculation at all. You just

*add the numbers together*. You don't need to know anything at all and you know you hit AC 14. You can't even get into a frame of mind where you might possibly think you need to subtract a bonus or add a penalty.

1. Roll the d20

2. Add your to-hit bonus to the result

3. Compare the result to the target’s AC to find out if you hit

to

1. Compare your THAC0 to the target’s AC to find out what number you need to roll on the d20 to hit

2. Roll the d20 to see if you hit

That's fine, but notice two things:

1. You're required to know the target's AC. This means either the DM has to do everything but roll the d20 -- and the DM is already doing

*a lot --* or the DM has to tell the PCs what the enemy ACs are and few DMs want to do that. Nevermind when there are modifiers and effects the PCs don't know about, like one person has a

*flame tongue* and they don't know the target is undead. Even when you know the target number on the d20, the DM often needs to know what you

*rolled.* Furthermore, it gets more confusing when you start attacking different targets with different ACs in the same combat because the target number on your die changes. As a player you have to remember the AC of each target instead of always rolling the same bonus.

*That* is why people prefer to just determine the AC you hit and then compare the AC you hit to the target. It's much more efficient to determine the target number on the monster listing rather than working backwards and determining the target number required on the die. The latter bakes in too many assumptions.

2. One of the steps in your ascending example is adding bonuses, and that's missing from the descending AC example. So you're skipping circumstantial bonuses entirely when you use THAC0. That's not realistic. No matter what you do, you've got to modify one of the numbers with your bonus. And you

*will* have circumstantial bonuses in 1e/2e! THAC0 doesn't eliminate the flanking bonus, or the facing-limited bonus shields grant, or the high ground bonus/penalty, or the bonus/penalty from

*bless*/

*bane* or

*prayer *or

*haste*, or the penalty from Paladins, or the racial bonus of Dwarves, etc. There are so many circumstantial bonuses in 1e/2e that you often deal with more than one at the same time. You can virtually never just write down your THAC0 as a single number and

*never* have to do math. That's a pipe dream. And since the system is based around the difference of the two numbers, a circumstantial bonus to number (die roll or THAC0) roll works exactly like a circumstantial penalty to the other number. You're right back in the same place of needing to be aware which number is which and knowing which value you're going to modify. Especially if you think of more circumstantial bonuses after you've done your roll, this can get confusing.

You can make descending AC faster. I would write down a THAC0 table on my character sheet when I played 1e/2e. Using a table is faster than doing the math in your head with THAC0, and it's much less error-prone. But... you're rolling the d20, adding your circumstantial bonus, and comparing it to a table you had to write down and have to maintain as you gain levels. You have to write down an entire table on your character sheet for each weapon you might use to make it fast. Or you can use a THAC0 wheel. But now all you're doing is automating the table. If anything, the existence of a THAC0 wheel or combat wheel proves how cumbersome the mechanic is. Can you imagine someone ever needing a combat wheel for ascending armor class? Even with 3e's own cumbersome and awkward design of iterative attack bonuses, nobody ever needed a

*game aid* to resolve a

*standard attack roll*. The very idea is absurd.