# OSROSR, Ascending AC or no?

## How critical is descending AC in an OSR product?

• ### I really don't care either way

• Total voters
49

#### Aldarc

##### Legend
There is also another method that hasn't been mentioned that doesn't fall into these options: roll under attribute.

The difficulty of monsters may affect the required TN of the attribute as well. You could even do the same for saving throws. You can see roll under with the OSR games such as Beyond the Wall & Other Adventures and Black Hack.

I’m not an OSR gal and I haven’t played any pre-3e editions of D&D (and accordingly, didn’t vote in the poll), but I will say this for THAC0: if you tell the players their target’s AC (I know, probably not something most OSR DMs are wont to do), it allows them to do the math first and figure out what number they need to roll to hit. This changes the process of rolling to hit from:
Letting the players know the Target Number is common practice for the Cypher System and Index Card RPG. ICRPG even just encourages a singular TN for difficulties in a room. So you may assign the room a TN (e.g., 13) and that is the AC of monsters and the difficulty of all checks in the room. You can further adjust this by making something in that room Easy (-3 TN) e.g., 10) or Hard (+3 TN: e.g., 16).

#### Helldritch

##### Hero
I voted for: "I don't really care either way".
One or the other, the result will be the same. You hit or you don't.
How you approach the matter is entirely a preference matter.

#### WayOfTheFourElements

##### Hero
Game mechanics don't make a game OSR or not. Descending AC makes perfect sense and the only flaw is how bad people are at basic math.
I now have strange desire to play Fate Accelerated as an OSR game.

##### Hero
I was excited by the introduction of ascending AC in 2e Dragon Fist and glad it became the standard of 3e onward. The descending AC calculations were always a mental speed bump in practice that I was happy to see reduced by ascending AC.

#### Sacrosanct

##### Legend
There is also another method that hasn't been mentioned that doesn't fall into these options: roll under attribute.
That's actually how I'm doing ability/skills checks (except rogues of course), as that's how we've always done it anyways going back to the early 80s. It's just easy, and also means every point of an attribute score matters if you have to roll d20 and get under the score in order to succeed.

#### vincegetorix

##### Jewel of the North
By the way, @Sacrosanct, keep us posted about the development of your game. Its seems to be something I would use at my table. I love the OSR games, but the ''git' gud'' and ''no true scotsman'' attitude of a part of the community makes it a hard sell to my players.

Your take with an eye toward inclusivity will be a breath of fresh air.

#### TwoSix

##### Unserious gamer
Supporter
I now have strange desire to play Fate Accelerated as an OSR game.
Just always max your Careful approach.

#### Snarf Zagyg

##### Notorious Liquefactionist
I prefer descending AC, and voted for it.

But I don't think it matters. That's not a dealbreaker.

Saving throws tied to abilities? That is a dealbreaker.

#### Ringtail

##### World Traveller
I'm a recent OSR convert and a big fan in general of OSR style games and Retro-D&D in particular.

I started with 4th and 5th Edition however and in general I find ascending AC easier to do quickly at the table.

HOWEVER, I have noticed that many OSR games are adopting a "Roll-Under" System, like The Black Hack. And when you look at B/X D&D it even recommends that ability checks, when they are rolled, be roll under checks. When you consider it in context, I can see how Descending AC makes sense as you want a certain level of consistency in your dice rolling mechanics.

I still prefer ascending AC however, though I don't mind either way. I also heard/read somewhere, that Descending AC produces slightly different probabilities? Don't quote me on that, I've no source.

#### vincegetorix

##### Jewel of the North
I must say, if the maths are done before the actual combat, and the ''to-hit'' table is made front and center, an integral part of the character sheet, with a ''to hit table'' for at least 3 weapons, I can see using descending AC, especially if there's not a bunch of possible modifiers. What I dont like is doing the THAC0 calculation at the table, every time.

That is, if you also use roll-under for skills. I'm a big fan of Beyond the Wall, but using different system for different actions made it confusing for my players at first.

Last edited:

#### Bacon Bits

##### Hero
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
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.

#### Ruin Explorer

##### Legend
@Bacon Bits brought up an interesting and fair point* in another thread, and I wanted to expand on it for it's own topic. I'll be creating a poll. For obvious reasons, this poll should be answered by people who are fans of the OSR as general rule, because the responses will hold the most value. I.e., if someone couldn't care less about the OSR votes, then it doesn't help me identify what OSR fans want. So thanks for respecting that request.

so the question: How necessary is it to have descending (THAC0) in an OSR D&D product that is not a direct clone of any one edition. Rather, a version that is simply meant to capture the feel and aesthetic of early D&D.

*Bacon Bit's quote:
Buddy, dude, man, you worded the poll and the thread title pretty much inversely from each other. I wish people wouldn't do that. That's going to have some wacky results, because some people will answer yes/no to the thread question without reading the poll - I know I nearly did!

Anyway, descending AC is just a bad idea that is simply an objective worse way of achieving exactly the same goal as ascending AC from a mechanical/numerical perspective. The only excuse to include it would be as an aesthetic choice to highlight how clunky and painful a lot of old-school systems are.

Descending AC makes perfect sense and the only flaw is how bad people are at basic math.
And yet it does move, as they say.

The fact is, D&D players are humans. Humans, period, are way worse at subtraction than addition, and even relatively smart humans sometimes make mistakes on subtraction that they'd never make on addition. This isn't opinion - this has been well-studied.

If this was an under-the-hood mechanic in a game being calculated with a computer, I'd agree, there's no particular reason to use one or the other (except when you display it to the player, because it's easier to understand that higher = better). But this is a mechanic people have to actually engage with. And people, in this case, has a 1:1 correlation with "humans" and humans are bad at subtraction.

#### Azzy

##### Newtype
2e was specifically made to be backwards compatible so descending was intentionally kept.
That was such a cop-out reason for why they didn't go ascending AC. All one would have had to do to use older monsters and NPCs was subtract their old AC from 20 and use the new attack bonus chart instead of the THAC0 chart. They could have even made an easy reference chart that equates the old ACs to the new ACs. That's the only thing that would have been done to use older material, and it requires less effort than some of the other 1e to 2e modifications you might need to do. And that's not to mention that 2e had compatibility issues in other regards (not least of all dropping some of the 1e races and classes).

#### Sacrosanct

##### Legend
By the way, @Sacrosanct, keep us posted about the development of your game. Its seems to be something I would use at my table. I love the OSR games, but the ''git' gud'' and ''no true scotsman'' attitude of a part of the community makes it a hard sell to my players.

Your take with an eye toward inclusivity will be a breath of fresh air.
I agree with you. Funny enough, I just posted a weekly preview thread here

I prefer descending AC, and voted for it.

But I don't think it matters. That's not a dealbreaker.

Saving throws tied to abilities? That is a dealbreaker.
I agree. It looks like ascending AC is the way to go. Saving throws are staying as they appear in TSR era D&D. Also, ability checks are roll d20 under your relevant ability score value. In case you're wondering

#### Sacrosanct

##### Legend
That was such a cop-out reason for why they didn't go ascending AC. All one would have had to do to use older monsters and NPCs was subtract their old AC from 20 and use the new attack bonus chart instead of the THAC0 chart. They could have even made an easy reference chart that equates the old ACs to the new ACs. That's the only thing that would have been done to use older material, and it requires less effort than some of the other 1e to 2e modifications you might need to do. And that's not to mention that 2e had compatibility issues in other regards (not least of all dropping some of the 1e races and classes).
I disagree. I've read Skip Williams' response to this, and I think it was the right choice. 2e went with a more campaign setting direction and got away from smaller adventure modules. Having folks have all of their older stuff work relatively seamless with 2e rules was the right choice. I also think you're overstating the ability of gamers to do that extra work. No, not ability, but willingness is a better word.

#### Ralif Redhammer

##### Legend
Of the things that give me happy OSR feels, descending AC isn't high on that list. Does it bring by some nostalgia, sure. But the utility of going to ascending AC and eliminating the middle step of THAC0 and attack matrices is of greater value.

#### Ancalagon

##### Dusty Dragon
Game mechanics don't make a game OSR or not. Descending AC makes perfect sense and the only flaw is how bad people are at basic math.
You would be surprised at how many people struggle with basic math. I would like to think that basic math skill is a lot less important than other things in whether or not someone should enjoy RPGs....

#### Snarf Zagyg

##### Notorious Liquefactionist
You would be surprised at how many people struggle with basic math. I would like to think that basic math skill is a lot less important than other things in whether or not someone should enjoy RPGs....
Aside-

I sometimes wonder if this is a chicken/egg thing. If you're an older gamer, you probably know that a lot (not all, but a lot) of your fellow "hardcore" TTRPGs went into fields that required a lot of reading, writing, math, and so on.

Correlation, causation, all that.

But I wonder if the amount of reps some of us got when we were younger with constant basic math and struggling with the purple prose of Gygax helped.

#### Aldarc

##### Legend
The Not-So-Subtle Subtext: "Did early D&D make us into better people than newcomers?"