Wednesday, July 15, 2009

Ascending vs. Descending AC

One of the things that struck me as a big improvement in 3e compared to prior editions is its ascending AC system. Back in the day, a lower AC was a better AC. You rolled a die, and either looked up the result on a chart or used THAC0 to figure out what AC you hit.

3e used an ascending system, where a higher AC is better. You rolled, added modifiers, and the result was the AC you hit. It seems patently better, but like a lot of things that changed over the course of D&D's history, it's better only within the larger context of the 3e rules, rather than in the context of D&D as a whole.

In 3e and 4e, there are lots of modifiers that go on top of that die roll. Not only do you have modifiers that apply to every roll, like ability score mods and magic items, but spells, conditions, flanking, and so on. There's a decent chance that 25% or so of the attack rolls you make during a session require some additional modifier beyond ability score and a magic weapon's plus.

In that situation, descending AC is a terrible idea. The table lookup or THAC0 math is just an extra step of work. Why not just use the final result?

However, strip away the fluctuating modifiers and the descending AC system comes into its own. At that point, all you need to do is record your to-hit numbers vs. AC on your character sheet. The process of roll and look up is, IMO, much faster when the players work up their own little attack matrices, faster than dealing with any math on the fly.

From the DM's side of things, you can do the same by tracking monsters on index cards. Just write down each critter's line of attack results, and you're done. The real drag with the system, IME, is using the table, but that's easily fixed.

As a side effect, I think this explains why the notorious weapon vs. armor type table in AD&D received so much flack. James Wyatt is the only person I've personally met who used it. It exacerbated the system's shortcomings and pushed it away from its strength.

The question then becomes, do you like lots of potential modifiers or not? And that, IMO, is a matter of taste.

There are a lot of little transitions like that between 2e and 3e, most notably the sudden explosion in power of spellcasters, that I think have a really big effect on D&D's direction in the past 10 years. In a lot of cases, the changes came about because of shifts in mechanics that have subtle effects on distant portions of the system. For instance, IMO the change in initiative made casters into unstoppable beasts, but that's another post for another time.

16 comments:

Oddysey said...

Oooooh . . . I've been using ascending AC in my Swords & Wizardry games because I started out with 3e so that's what I'm comfortable with. At some point, though, I've wanted to try out descending AC, if only for the old school experience of it. Knowing this, that experiment will be much more successful.

Dave The Game said...

That is one of the concerns I have with 4e that I had with 3e as well- tracking small niggling bonuses, especially situational ones. (3e was worse for me because a number of those bonuses cascaded). I would much rather see a smaller selection of larger bonuses to attacks, since the big ones are much more likely to be remembered, and if they're more rare, they're more likely to be remembered in play.

I went many years with THAC0, and my group was never a fan of cross-referencing on charts and such.

Jer said...

However, strip away the fluctuating modifiers and the descending AC system comes into its own.

Speaking as someone who figured out sometime around 1985 how to turn the descending AC system into an ascending AC system and hasn't used anything else since, I STRONGLY disagree with this. The ascending AC system is even better when you don't have fluctuating modifiers to worry about. You roll a die, add a single number, and you know what AC you hit. What could be more intuitive than that? And if you really want to use an attack matrix line to simplify attack rolls you still can do it just as easily with the ascending AC as you can with descending AC.

I honestly don't see any benefit to using the descending AC system beyond some level of nostalgia that I just don't have because I haven't used descending AC for almost 25 years. Since everything you have here as a benefit of the descending system can be implemented in the ascending system just as easily, with the side benefit that for folks who find "roll a die and add a number" more intuitive than "use an attack matrix lookup", I'm still not seeing it.

Mike Mearls said...

Jer - The real benefit comes in when you take the time to work out matrices ahead of time. On the fly, the table is ponderous. If you have everything prepped, I think it's much faster.

For instance, if you have an index card for each monster type with that creature's rolls vs. AC plotted out, I think things move much faster for the DM. Now, that assumes you don't have many (or any!) common modifiers out there other than things you can account for when you build a specific NPC or creature's matrix, like flanking. As soon as I'm adding situational modifiers more than 25% of the time, I think ascending is clearly superior.

Dave - There are times when I wish combat advantage was the only on the fly attack modifier in the entire game!

Oddysey - Like I mentioned above, I think it works best if you work up index cards for each monster type that you're using and have the players note their target numbers on their character sheets.

Dar said...

If you remove those fluctuating modifiers, aren't they the same? Couldn't you as easily work up a little table with ascending AC?

I depend on those modifiers to adjust game play on the fly for all sorts of reasons, from rewarding good play, to adjusting bone headed encounters of my creation. They are an essential tool. I'd rather not do without them.

Andreas Davour said...

Having read all the posts today about Jonathan Tweet and the flood of responses I must say that this post of yours was one of the few that didn't make me cringe.

I disagree with almost everything you did as a designer of 4th ed, but you don't come across like a smug jerk on this blog, like Jonathan. Thanks Mike. I Needed that.

Now if only my fellow old schoolers could stop talking about AC...

Brock Cusick said...

"There are a lot of little transitions like that between 2e and 3e, most notably the sudden explosion in power of spellcasters, that I think have a really big effect on D&D's direction in the past 10 years. "

I think the addition of Feats and changes to the Multiclassing rules (including Prestige classes) was the far bigger change. It made "Pimp my Build" the game within the game and gave decisive in-game advantage to good builders. Up through AD&D 2E the D&D game had been one of fixed Archetypes and focused on the in-game aspects of playing your character rather than the meta-game aspects, but 3E took that away almost entirely. Changes like ascending AC were must less important.

buzz said...

"On the fly, the table is ponderous. If you have everything prepped, I think it's much faster."

I'd rather have a mechanic that was intuitive than one I have to prep a matrix for in order to make usable.

Ascending AC is intuitive. Descending AC is something you get used to. I like a lot of things about Old School, but fetishizing descending AC (and similar silly mechanics) is not one of them.

Jer said...

For instance, if you have an index card for each monster type with that creature's rolls vs. AC plotted out, I think things move much faster for the DM.

Hm. I guess. But I always found that ascending AC not only cuts my prep time quite a bit on its own, but it also makes improvising when players go off track or for unplanned pickup games much easier for me. There's not much prep that you need when you have the attack bonus and modified AC penciled right into the monster write-up - you can run it all right from the book.

James V said...

When descending AC can be stramlined as you described, it can be pretty fast, what I have always found facinating is how it is seen as lacking in logic.

Maybe this little piece here can help shed a little more light on those who don't see the logical explaination for descending AC.

The higher your chance of being sucessfully attacked = Higher AC

The lower your chance of being sucessfully attacked = Lower AC.

Think of things in that light, and it makes perfect sense. I think the only true barrier to understanding was framing the magic armor bonuses in the same terms as magic weapons, with plusses. That inconsistency in how you understanded AC with how the rules chose to calculate its adjustments throws people for a loop.

yeloson said...

One place where a matrix could make sense might be like the Saving Throw chart- it had interesting things like bell curves and plateaus in it- classes that started out as the worst in something sometimes became the best by the end of it.

But that's a pretty specific design choice, and I don't know how much thought went really into it throughout the editions that used it.

polanimation said...

In 4E, each monster having four different defense-numbers actually takes away from the speed-advantage of ascending AC.

Having 5 monsters in the encounter, for potentially 20 different defense-numbers, makes checking if the roll is high enough into a table lookup anyway.

(The change in initiative for a large part being responsible for spellcasters being overpowered is an awesome insight, by the way.)

zipdrive said...

Are you guys serious?

There is absolutely NO mathematical difference between the ascending vs. descending AC. It's only a matter of convenience.

Mike, you can make a chart and use all the index cards in the world with the ascending system just as well as a descending one. Just, as you say- include all the known modifiers in advance (or ignore them).

In addition, where were these sentiments when you were designing 4E? One of the things I remember is reading a preview article with one of the 4E designers (can't remember who) who said one of the goals was to streamline the action and reduce the number of dice rolled (hence no crit confirmation, no effects that occur for X rounds, and the simple division of at-will/encounter/daily powers).

Only then you dropped the ball: 4E has twice-in-an-encounter powers, complicated until-the-end-of-your-next-turn effects and a billion modifiers to keep track of (e.g. having a fighter and paladin mark flanked targets on a difficult terrain with a warlord casting buffs in the neighborhood, and that doesn't include the monsters' powers).

Why?

Lizard said...

a)You could just as easily write down all your 'to hits' on your character sheet using an ascending AC, too. :) And not have to explain that "+2 Armor" meant your Armor Class was two LOWER.

b)I like lots of little modifiers. I like to feel I have tactical choices which make a difference in combat. I think a "largest modifier only" system might be a good compromise between flexibility and simplicity, and avoid people trying to eke out every little +1 they can. OTOH, it can lead to stale gameplay and people obsessing over getting one big bonus instead of two smaller ones.

Noumenon said...

Just for anyone else who comes upon this post, Mearls discusses the hint in the last paragraph here.

AzaLiN said...

Okay, flaming aside, has anyone here used descending in Swords and Wizardry effectively? I had an unexpectedly large group yesterday starting my campaign - Keep on Borderlands, I found a copy at a used book store!- so I quickly abandoned my attempt to make them learn a new AC system, but I'm still pretty curious. It clicks with me more, but I don't remember well enough how smooth and adaptable it is.