If AC is noted as the difference from unarmoured (i.e. the bonus in modern editions), then it can easily be made into a roll-between system (rolls that are below the attacker’s stat but higher than the target’s AC hit).
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Just to clarify - even with descending AC you are still trying to get a high roll on your d20 with attacks.
I like THAC0 and it’s really easy once you get it (or write the proper matrix row on your character sheet) but you’re still rolling high to hit a low AC. I just don’t think it’s hard to remember that with attacks 20s are good whereas I do get the ability checks and saving throws messed up.
You could invert the THAC0 matrix and have everything be roll-low, but rolling for a 1 instead of 20 on attacks feels off to me 
I think simply changing the saving throws to roll-under is enough to reduce confusion without being disruptive.
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Ha, yes the ascending/descending thing gets me mixed up. But then, couldn’t you roll under the AC? So if unarmored I’m AC 9, you need to roll a 9 or less to hit me, with your strength bonus decreasing your d20 result? You need to roll under your strength for a STR check, and you need to roll under your inverted-10 saving throw. All roll under now?
Instead of subtracting your to-hit bonus, why not just add it to the enemy’s AC?
(Granted, everyone needs +2 bonus to have a 50% chance of hitting AC 9.)
The easiest way would be to invert the THAC0 matrix if you wanted to implement a roll-under system because it’s already “balanced”, otherwise you’d have to do a lot of work ensuring everything matches.
For example, let’s look at the THAC0 table assuming a THAC0 of 19 (or +0 to hit if we’re using ascending AC):
| |Attack Roll to Hit AC
|Thac0| -3 | -2 | -1 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
|19[0]| 20 | 20 | 20 | 19| 18| 17| 16| 15| 14| 13| 12| 11| 10|
The above is saying that in order To Hit AC 0 you’d need to roll a 19 (aka THAC0 19). To hit an unarmored creature, you’d need to roll a 10 To Hit AC 9. You’ll notice that your armor score can actually go to negative numbers. At THAC0 19 you would only hit a negative on a 20, but as you level up this changes - see the below example for THAC0 14 (or +5 to hit with ascending AC):
| |Attack Roll to Hit AC
|Thac0 | -3 | -2 | -1 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
|14[+5]| 17 | 16 | 15 | 14| 13| 12| 11| 10| 9 | 8 | 7 | 6 | 5 |
You’ll see in this case you only need to roll a 17 To Hit AC -3, or a 5 To Hit AC 9 (unarmored).
Now, if you wanted to change this to roll-under, you’d simply reverse it. Our first example for THAC0 19 or +0 to hit would be:
| |Attack Roll to Hit AC
|Thac0| -3 | -2 | -1 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
|19[0]| 1 | 1 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10| 11|
In this instance you’d have to roll a 1 to hit negative ACs, or an 11 to hit an unarmored target. Our other example of THAC0 14 or +5 to hit would look like this:
| |Attack Roll to Hit AC
|Thac0 | -3 | -2 | -1 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
|14[+5]| 4 | 5 | 6 | 7 | 8 | 9 | 10| 11| 12| 13| 14| 15| 16|
You’d have to replace all these tables to make descending to-hit work. I think it’s more effort than it’s worth since the confusion would be alleviated with the simple change I mentioned above of inverting the saves.
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This is sort of what I do. I use the normal ability modifiers, allow players to roll a d20 and I set a Target Number based on the situation. Something pretty simple, make it a 10, a challenge, make it a 15, something serious, make it an 18. I use the usual 1 is a fumble/20 a massive success, etc.
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Hmmm, this is actually interesting. For ability checks just d20 + ability, and try to beat a target number. For average difficulty it would be a 20, then progress upward…22, 24, 26, etc.
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I’ve experimented with this and I dig it. But, my concern was the math treads on “I dun wanna” territory. Like, adding 13 + 18, while not hard, might be just big enough numbers to dissuade less-mathy players (or younguns) from enjoying the mechanic.
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I have thought a little bit about this before, and a solution might be:
treat stats as descending from the beginning, meaning you flip the modifiers just around, like this:
| Ability Score | Modifier |
|---|---|
| 3 | +3 |
| 4-5 | +2 |
| 6-8 | +1 |
| 9-12 | 0 |
| 13-15 | -1 |
| 16-17 | -2 |
| 18 | -3 |
And then you call them something like feebleness, clumsiness, frailty, foolishness, ignorance dullness, so it makes sense that when the number is higher = it’s worse for the character.
but also this kind of doesn’t feel good to define a character only by his negative traits, I think.
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When I GM B/X (OSE) I never use ability checks. I don’t want a random non-choice make that much impact on the game. I just tell them what X-in-6 or d% chance they have and they can negotiate, roll or decide to do something different.
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I get that, but stat checks just feel kind of nice, idk. Like, I really like to do dexteritychecks once in a while, when players try to do acrobatic tricks
I find them reductive and empty. I like the x-in-6 because it creates dialogue and negotiation when our visions of the difficulty of something doesn’t align. Calling for a check doesn’t really help explore the game fiction the same way negotiating a roll does. I also dislike reducing a complex situation to “how good is your Dex?”.
fair enough, I just feel that x-in-6 checks are a bit too loosely related to the characters the players have. What do you mean by negotiation?
There are some games where the trick is to roll as high as you can but still under your score so that might be the compromise you want to do
The x-in-g takes into account their abilities. Negotiation is like:
DM: “Ok, I’ll give you a 1-in-6 chance of opening the crate”
PC: “Well I do have a 16 Str and a crowbar and a hammer.”
DM: "Ok, then a 3-in-6 chance of forcing it open with the crowbar or you can smash it open with a hammer and I’ll roll to see if there’s an additional random encounter from the noise.
PC: “What if I cover the hammer with a cloth and use it to tap the crowbar in between the lid and the box and see if I can pry it loose that way?”
DM: “Then you won’t have to roll and you just get the one random encounter check for the time spent.”
So, I see I’m not the first person to resurrect this thread, but apologies for being late to the party. I just wanted to point out that @YnasMidgard’s suggestion to do roll + ability >= 21 is the same approach as Delta’s “Target 20” approach to to-hit rolls, found here: https://deltasdnd.blogspot.com/2009/07/what-is-best-combat-algorithm.html
In that post he goes through some of the research behind what kinds of mental math folks can do most readily (might be different for kids), but the tl;dr version is that addition is easier than subtraction and that comparing to a constant target number every time is easier than different target numbers every time.
I’ve been playing with this method for years, but still use roll-under ability scores. I don’t know why I didn’t make the connection and do something like @YnasMidgard mentions.
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Part of the resurrection crew as well - just wanted to say I appreciate all the work done in this thread. Legitimately going to be referring to this in the future for many different reasons.
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I like this:
Ability check: roll 3d6 + ability modifier. 11+ is a success. Hard checks are rolled with 2d6, easy checks with 4d6.
It’s roll higher.
It’s a simple + mod.
It can be scaled without adding numbers.
You can use dis/advantage if you want smaller scaling.
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I’ve been leaning on the Turn Undead table for Task Resolution lately. It solves a lot of the issues I have with the Optional Ability Check 
- It accounts for Character Level
- It accounts for Task Difficulty
- Rolling High Is Good
- Delicious Bell Curve for Competency over time/Additional Dice Metadata (d36, d6 tables!)
- It features “Degrees of Success” that are not really present outside of a few other places (such as the Reaction Roll, which is probably a close cousin to this table)
- It places Dice in the Player’s Hands/Allows for Obfuscated Resolution (so you don’t have to roll “Find Traps” for the Player in order to prevent the whole “witness failure in real-time”)
- It allows the Character to Attempt things outside of their normal Level
I go into in depth here: d4 Caltrops: RC Hacks & House Rules pg 15: Turning Undead Table (mostly as a replacement for Thief Abilities, but it’s proved useful for other situational things where consequences are interesting and random chance is a factor).
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other than adding an extra die for 4d6 and that it’s 12 or more not 11 or more - that’s the central WHITEFRANK mechanic for all rolls