# D&D GeneralThe Rules Cyclopedia - Unlearning Dnd Preconceptions from a 3e player

##### Legend
Quick: what’s 17-12?
17 - 10?
17 - 8?
17 - 19?

congratulations, you just used THAC0 to work out the armour hit 4 times in quick succession. That’s all there is to it.

If you have a +1 to hit with dex this turns into two options

Take your +1 to hit and pre modify your THAC0 by subtracting the +1 from the THAC0 number to get the number you need to hit AC 0.

So

16-12
16-10
16-8
16-19

So it is still a one factor calculation.

Or you can add the +1 to the attack roll as a bonus to the second part to figure out the roll.

17-(12+1)
17-(10+1)
17-(8+1)
17-(19+1)

both work mathematically but the first is a little more straightforward on the fly.

A PC can reasonably choose whatever way they want and get to the same result.

However in B/X, BECMI, RC you also have situational things like your ranged attack probably being at short range for +1 on that specific attack.

So the two options then look like this:

16-(12+1)
16-(10+1)
16-(8+1)
16-(19+1)

17-(12+1+1)
17-(10+1+1)
17-(8+1+1)
17-(19+1+1)

You could alternatively add in the +1 to attack as an on the fly minus to the THAC0 part of the equation for more options

(16-(+1))-12
(16-(+1))-10
(16-(+1))-8
(16-(+1))-19

(17-(+1))-(12+1)
(17-(+1))-(10+1)
(17-(+1))-(8+1)
(17-(+1))-(19+1)

For ascending AC It becomes

12+3+1+1
10+3+1+1
8+3+1+1
19+3+1+1

Or if they combine their dex mod into their ranged attack on their sheet it becomes

12+4+1
10+4+1
8+4+1
19+4+1

In ascending AC the DM can ask what did you roll and what are your bonuses and all the factors are captured.

In THAC0 the DM can ask what did you roll and what is your THAC0 and that might or might not capture all the relevant modifiers depending on how the calculation is (validly) being done.

#### transmission89

The math involved is counter intuitive, and so it is hard to remember what the exact rule is, regardless of how simple it is.

I think the main confusion comes from 3 things:

The negative armorclass system:
It seems nonsensical that the lower your armor score, the better the defense. What makes it worse, is that we're basically dealing with both values above 0 and below it. THAT is confusing. Why does someone with no armor on, not have AC 0? But no, base AC is 10 in 2E. They could have just had it scale from 0 to -10, but instead for some weird reason it is from 10 to -10. So a character in 2E starts from AC 10. Then, as they add pieces of armor, their AC gradually lowers until it is below 0.

Thac0:
The fact that this isn't a straight attack bonus, but that you substract your roll from the Thac0 score is weird. Also, naming it Thac0, and not "attack", makes it further confusing.

While both are simple math, adding numbers is easier and more intuitive. Further more, it is easier for many people to compare positive numbers, rather than negative numbers.

So it is both confusing, and hard to remember, and unintuitive. And as I'm writing this down, I find myself rereading it just to make sure I got the rule right.

Indeed, I agree, everything together made it oblique, which is why it was changed in 3e. But again, still not a confusing bit of maths.

The negative armour class system: Already explained above

Calling it THAC0: I mean, it literally says what it does on the tin (to hit armour class 0). But again, acronyms to those not in the know can be intimidating.

Adding is easier than subtracting: I’ve already discussed above that the operational time has minimal impact within the number range of the game.

#### transmission89

If you have a +1 to hit with dex this turns into two options

Take your +1 to hit and pre modify your THAC0 by subtracting the +1 from the THAC0 number to get the number you need to hit AC 0.

So

16-12
16-10
16-8
16-19

So it is still a one factor calculation.

Or you can add the +1 to the attack roll as a bonus to the second part to figure out the roll.

17-(12+1)
17-(10+1)
17-(8+1)
17-(19+1)

both work mathematically but the first is a little more straightforward on the fly.

A PC can reasonably choose whatever way they want and get to the same result.

However in B/X, BECMI, RC you also have situational things like your ranged attack probably being at short range for +1 on that specific attack.

So the two options then look like this:

16-(12+1)
16-(10+1)
16-(8+1)
16-(19+1)

17-(12+1+1)
17-(10+1+1)
17-(8+1+1)
17-(19+1+1)

You could alternatively add in the +1 to attack as an on the fly minus to the THAC0 part of the equation for more options

(16-(+1))-12
(16-(+1))-10
(16-(+1))-8
(16-(+1))-19

(17-(+1))-(12+1)
(17-(+1))-(10+1)
(17-(+1))-(8+1)
(17-(+1))-(19+1)

For ascending AC It becomes

12+3+1+1
10+3+1+1
8+3+1+1
19+3+1+1

Or if they combine their dex mod into their ranged attack on their sheet it becomes

12+4+1
10+4+1
8+4+1
19+4+1

In ascending AC the DM can ask what did you roll and what are your bonuses and all the factors are captured.

In THAC0 the DM can ask what did you roll and what is your THAC0 and that might or might not capture all the relevant modifiers depending on how the calculation is (validly) being done.
Yes, modifiers exist. Which is why it’s better expressed as AC hit = THAC0 - (roll + mods) so you’re only subtracting one number. That way leaves no difference to ascending ac adding multiple modifiers.
I will also state that I certainly don’t think the rule books helped with their ham fisted explanation of THAC0. It was a botched, convoluted explanation. The actual sub system itself? Not complex.

#### teitan

##### Legend
I never played 3e, but I played 1e / BECMI, 4e, and now 5e. In none of those editions (even 4e) are miniatures required. I have played with and without them in each of those editions.
No, they aren't required but the system for 3.5 and 4e is explicitly designed for use with a battle map and some sort of markers, so much so that the Essentials sets came with markers to be used with the battlemats and the Monster Vault had counters for every monster variation. Yes you can play without them but the system is designed for them and explicitly so. 1e & BX/Mentzer Basic mention minis but do not go over them in depth beyond being handy tools. When the system is designed and says "you can move this many squares in a turn" then, while your argument is certainly valid, your argument was not the expected norm. 3.5 and 4e very explicitly call for miniatures with counters being a secondary unmentioned option once WOTC was not putting out D&D Miniatures until Essentials. You can keep saying "but I didn't use them" and that's perfectly valid but does not reflect the rules as written.

#### teitan

##### Legend
It's both.

Look, I've played 2E for a very long time, and I loved it. When 3E came out, I strongly resisted moving to the new edition. Me and my friends hated having to buy all new books again, and learn all new rules. We thought 2E was fine and that we didn't need a new edition.

Then we played ONE test game of 3E, and never moved back to 2E. Not having the thac0 system and reverse armorclass was such a relief. None of us could ever quite remember how it worked. It had to be re-explained every single time we made an attack. Asking our DM what AC we hit became the default mode of play, it was that bad, and terribly frustrating on our DM.
I am with you on this while agreeing with Transmission. 3e was just plain easier to explain to people. I don't think Thac0 was hard at all, like his argument. My players kept a chart on their sheet that showed what their roles would hit going from 10 to -10 and adjusted it when they leveled up but once 3e came out and it was oh I rolled an 18 to hit, it was a lot more obvious and at a glance to know if you succeeded. 3e was overall a better game in general because it was cohesive and addressed a lot of the issues discussed in this thread when it comes to the older editions. To me it seemed like 3e was Basic with a lot of house rules made official and a cleaned up, consistent skill system. MY long term issue was that 3.5 became a bear to run with monsters working like PCs (made sense at the time) and the complications that were templates and ever escalating numbers.

#### dave2008

##### Legend
No, they aren't required but the system for 3.5 and 4e is explicitly designed for use with a battle map and some sort of markers, so much so that the Essentials sets came with markers to be used with the battlemats and the Monster Vault had counters for every monster variation. Yes you can play without them but the system is designed for them and explicitly so. 1e & BX/Mentzer Basic mention minis but do not go over them in depth beyond being handy tools. When the system is designed and says "you can move this many squares in a turn" then, while your argument is certainly valid, your argument was not the expected norm. 3.5 and 4e very explicitly call for miniatures with counters being a secondary unmentioned option once WOTC was not putting out D&D Miniatures until Essentials. You can keep saying "but I didn't use them" and that's perfectly valid but does not reflect the rules as written.
Squares are no less daunting to imagine than inches back in 1e. Yes I agree the rulebooks were written with the assumption of a grid and minis, but again nothing in those rules requires them. We played 4e with 6 PCs and me as the DM in two 6 hr card rides (well minivan really). This was a lvl 30 adventure and we had no minis or battlemap and add to roll our dice in a box. It was one of the most epic adventures I have ever run.

#### teitan

##### Legend
Squares are no less daunting to imagine than inches back in 1e. Yes I agree the rulebooks were written with the assumption of a grid and minis, but again nothing in those rules requires them. We played 4e with 6 PCs and me as the DM in two 6 hr card rides (well minivan really). This was a lvl 30 adventure and we had no minis or battlemap and add to roll our dice in a box. It was one of the most epic adventures I have ever run.
Also beside the point in comparing 1/2/B editions in contrast with the rules assumptions of 3.5 & 4e. Sure, you're right and you keep posting these comments on every thread like this but they are beside the point being draawn. It's the D&D equivalent of "what aboutism". It happens and no one is denying it but in contrast it is not the default.

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