I'm aware of at least two games (Index Card RPG and Machinations of the Space Princess) which use constitution, rather than dexterity, as the attribute which determines a character's melee AC or defense. In both cases, this appears to be primarily a gamist concern. By that I mean that the choice appears to have been made largely to balance out the usefulness of the six attributes. That makes some sense to me, as Dexterity tends to be dangerously close to being a super-stat in D&D and similar games. It usually affects defense, ranged attacks, intitiative, stealth, and saves against the most damaging spells. Whereas Constitution usually just grants a small bonus to hit points, and a bonus against certain spells and effects. In Index Card RPG, it doesn't even help HP.

That's unremarkable. Thing is, I'm beginning to think it makes a certain amount of sense from a simulationist perspective as well. This seems counterintuitive. I mean, no amount of being tough will prevent you from being hit by an axe, right? Well, D&D and related games are trying to simulate medieval combat, mostly between people wearing some level of head-to-foot armor. We don't really know what that looks like. The best modern analogues we have are HEMA, which pretty much universally assumes not only no armor, but that one hit ends a fight, and things like M-1/IMCF or Battle of the Nations, which do involve armor, but are still fundamentally about people trying not to kill each other.

I've been watching a lot of combat sports lately, MMA and Muay Thai particularly, and even though they're unarmed, I still think they're extremely valuable for understanding real combat. One thing that stands out is how rarely strikes are parried, at least in the sense we're used to parries in books and films. A fighter with great defense is more often one that knows how to either not be there to be hit at all (either through distance or positioning), or one who knows how to get "hit", but in a way that doesn't actually do any damage. With a few notable exceptions, fighters who are known for how rarely they get hit are often that way because they're so effective offensively that their opponents never get good opportunities to attack them. This complies with my own limited martial arts experience, and much greater experience with fencing and HEMA. You do get exchanges of multiple blocked blows, but they're much rarer than a period of jockeying for position, followed by one or the other fighter getting in an unparried hit. At a basic level, the attacker always has the speed advantage, in that the defender has to perceive the attack and react to it before they can even start moving.

The part of combat defense that comes down to distance and positioning could arguably be part of Dexterity, but since it's as much a function of experience as anything, it could equally get filed under Wisdom or even character level. But I'm starting to think that Constitution as a representation of a fighter's ability to roll with blows, catch them on their armor or otherwise mitigate the damage, is a lot less unrealistic than it appears on the surface. After all, AC is not a representation of "how hard you are to hit", but rather "how hard you are to wound".

Anyway, this is more a rumination than a question or an argument, but I'd be curious to see what others think of the idea.

It's not completely out of left field.  However, I would suggest that if balancing the stats is the primary motivation, a better move would be to merge Constitution into Strength and then split part of Dexterity out into Agility. 

You can rationalize a lot of different things in order to keep the standard D&D 6 scores, but at some point all the secondary effects start to add up in ways such that it is better to make a clean break.  Though I think the Constitution as defense works better with some D&D editions and spin-offs than others.  It's a substandard choice when using skills. Yet the change is not as useful in editions where the Fighter types are already more capable.  Because part of what makes Dexterity over-powered is being picky over some aspects of simulation while selectively ignoring others.

Finally, another argument against is that Constitution already helps with "not getting seriously damaged" because it piles on the hit point buffer.  Using it for AC could be double-dipping.  This argument isn't as strong to me. Other places in D&D double-dip, as you would expect in a game with a lot of archetypes.  Plus, I would say that the main issue there is out of control hit point scaling in some editions, not Constitution specifically.  There's other ways to mitigate the excessive scaling. 

It's actually fine for 5e D&D, because everyone needs Con as it is, and Dex is a superstat in 5e, and this would take away part of what makes it so annoying by homogenizing it with Str some.

But in 3.X, or OSR?  I don't think so.  I think the real thing is, do you have +Dex to damage?  If so, it's too good, given all the other things.  3.X and Pathfinder 1 didn't have this (there was some obscure Pathfinder crossbow thing- that doesn't count, no).

In OSR I don't think it's a good idea.

There's also one more detail- it's quite plausible that it should be able to help you dodge missiles, but Con should help you in other situations.  Games generally don't have this as a valid detail, but, hey, it's there.

I consider "constitution" to be like "Toughness" in Magic: The Gathering.  In my system, I use a stat, "Endurance", that is used as defense value against an attack.

Quote from: ForgottenF on March 15, 2023, 08:56:35 PMDexterity tends to be dangerously close to being a super-stat in D&D and similar games. It usually affects defense, ranged attacks, initiative, stealth, and saves against the most damaging spells.

That's true, but in D&D and similarly wide-range-of-level-development games, it's generally less of an edge in practice than it might imply once level gap is sufficiently high to compensate.  A 5th-level fighter with DEX 11 is still going to outroll, on average, a 1st-level fighter with DEX 15 on most of those rolls.

And class focus and progression will also affect how much use a non-fighter character can get out of those scores; a wizard with DEX 17 is going to be harder to hit than a wizard with DEX 11, but a first-level fighter in plate mail is still going to be harder to hit than either of them, and armour you can wear has a much greater development range than attribute scores.

QuoteI've been watching a lot of combat sports lately, MMA and Muay Thai particularly.... One thing that stands out is how rarely strikes are parried, at least in the sense we're used to parries in books and films. A fighter with great defense is more often one that knows how to either not be there to be hit at all (either through distance or positioning), or one who knows how to get "hit", but in a way that doesn't actually do any damage.

Also correct, but the problem here is that taking a punch or kick, and taking a knife thrust, arrow strike or sword swing -- or, for that matter, even taking hits from other blunt-damage weapons like clubs or staffs; leverage and momentum are devastating things -- have very different effects.

This could be modelled by giving CON an AC bonus that applied only to blunt unarmed damage, but that still wouldn't reduce the desirability of DEX and armour as a way to reduce the likelihood of taking any kind of damage, especially if the bonuses didn't stack (and if they did stack, someone with both high DEX and CON might well become effectively unhittable to anyone without a serious level advantage).

It's a reasonable complaint to point out that a combatant who is merely fast shouldn't be able to consistently outfight someone who's strong, tough, and fast, which is where objections to DEX as a combat "super-stat" come from. But as long as you have room for sheer experience to more than offset all of these, I think as a game it's still adequate. (Nobody has ever complained about INT being the dominant "super-stat" for arcane magic, after all.)

Quote from: Stephen Tannhauser on March 16, 2023, 02:50:01 PM
That's true, but in D&D and similarly wide-range-of-level-development games, it's generally less of an edge in practice than it might imply once level gap is sufficiently high to compensate.  A 5th-level fighter with DEX 11 is still going to outroll, on average, a 1st-level fighter with DEX 15 on most of those rolls.

Well, not in 5th edition he won't, because the 5th level guy's proficiency only is 1 higher than the 1st level guy's proficiency, so it's a +4 for the 1st level dude versus a +3 for the 5th level.  The 1st level guy also is getting +2 to his damage rolls here, on top of the extra accuracy.
In 4th edition, they are tied, as 5th level guy gets a bonus of +2 (half his level) and the 1st level guy gets +2 (from his Dex).
In 3rd edition, the 5th level fighter has the advantage because he has +5 from his base attack, while the 1st level dexxyboi gets +1 from base attack and +2 from Dex.
In AD&D it's not even an issue- a Dex 15 doesn't get shit for hit bonuses.  In older editions it's also definitely not a superstat either.

Anyway your case applies only to 3rd edition.  It's also not the point though- the point is that, if you're a player creating a character, you don't want to feel like your strong hero is just a mechanically not supported choice compared to the agile hero.  The versions of the game that tie every goddamned thing to Dex definitely do push Dex a lot.  Most 5e players (the version hardest hit by this) don't want to give up their +Dex to damage, so instead try to come up with fixes for Strength.  I don't think there's any house rule with a lot of support even among those with the idea though.

QuoteAnd class focus and progression will also affect how much use a non-fighter character can get out of those scores; a wizard with DEX 17 is going to be harder to hit than a wizard with DEX 11, but a first-level fighter in plate mail is still going to be harder to hit than either of them

Sure, but a 1st level fighter doesn't have plate.  But again, the point is that with that Dex of 17 the wizard will get a +3 to hit with ray spells in 3rd, and have it as a bonus to range attacks in 3rd, 4th, and 5th.  He'll also be a lot more accurate with a ranged weapon in older editions as well, because 17 Dex is high enough for bonuses in AD&D (and a +2 bonus at that!).  Later, he'll write a book, A Wizard And His Darts about how he didn't even need to cast spells until 5th level because of bullshit "advanced" dart math.

Sure. You can make arguments for it. I mean, the very best argument I can think of in favor of it would be that bobbing and weaving can get pretty exhausting. Assuming you're pacing yourself, stamina may be more of a limiting factor than agility when it comes to defense.

The problem with that is, it's not immediately obvious to me that a) the Dex defense bonus is due to bobbing and weaving rather than quick reflexes, and b) that requisite stamina is not already built-in to Dex. For instance, there's a certain finger stamina when you're shredding righteous licks on the electric lute, but that finger stamina does not necessarily correlate to stamina overall or general health or well-being.

So, I think the non-overthinking take is probably the best one in this case, and therefore I rate this idea:

I think this makes a lot of sense. I really dislike HP bloat created by con bonuses, and can absolutely see the merits to this. And when fighting...endurance, stamina and toughness, the ability to take a hit, is just as important as the ability to land one.

Well you could just say your Con bonus mitigates direct damage?

Remember in 1e Unearthed Arcana Full Plate and Half-Plate armor did this?

It is an extra calculation, so take that into consideration. 5e's chonky HP totals would make combat that much more long and tedious. But for leaner sorts of games it might fit the bill with very little effort.

Quote from: tenbones on March 17, 2023, 10:13:16 AM
Well you could just say your Con bonus mitigates direct damage?

Remember in 1e Unearthed Arcana Full Plate and Half-Plate armor did this?

It is an extra calculation, so take that into consideration. 5e's chonky HP totals would make combat that much more long and tedious. But for leaner sorts of games it might fit the bill with very little effort.

It's really starting to stretch the bounds of verisimilitude, but if otherwise in love with the idea and want to handle that, then instead add the Constitution mod to melee damage (possibly excluding the smaller melee weapons).  The rationalization would be that the person with that extra stamina can dish it out closer to full power, without exhausting themselves.  Damage is speed and power, which is mostly Strength, but most people can't go full out for very long.

Would hate that in B/X and AD&D, but in 5E most of the negatives turn to positives.  Cuts down bloated hit points faster, redresses some of the problems with Dex adding to damage, and is no extra handling time, since it goes right onto the weapon line on the character sheet.  I mean, it would be better to lower hit points across the board and cut out Dex damage, but this is a sneakier way to get some of the same effect.

In D&D, taking the other guy out fast is damage reduction.

Well... if I had my way, you'd get max class HP + Con bonus, and 1hp/lvl +Con.

And that's it. I'd factor everything else in Defense + Armor values on the back end, which is probably another discussion.

Quote from: tenbones on March 17, 2023, 02:14:44 PM
Well... if I had my way, you'd get max class HP + Con bonus, and 1hp/lvl +Con.

And that's it. I'd factor everything else in Defense + Armor values on the back end, which is probably another discussion.

Oh, wouldn't be my first choice.   Seems to be mitigating problems instead of fixing them.  I can see it in someone wanting to quickly house rule a system without changing any of the existing abilities, though.

Quote from: Stephen Tannhauser on March 16, 2023, 02:50:01 PM

Quote from: ForgottenF on March 15, 2023, 08:56:35 PMDexterity tends to be dangerously close to being a
I've been watching a lot of combat sports lately, MMA and Muay Thai particularly.... One thing that stands out is how rarely strikes are parried, at least in the sense we're used to parries in books and films. A fighter with great defense is more often one that knows how to either not be there to be hit at all (either through distance or positioning), or one who knows how to get "hit", but in a way that doesn't actually do any damage.

Also correct, but the problem here is that taking a punch or kick, and taking a knife thrust, arrow strike or sword swing -- or, for that matter, even taking hits from other blunt-damage weapons like clubs or staffs; leverage and momentum are devastating things -- have very different effects.

This could be modelled by giving CON an AC bonus that applied only to blunt unarmed damage, but that still wouldn't reduce the desirability of DEX and armour as a way to reduce the likelihood of taking any kind of damage, especially if the bonuses didn't stack (and if they did stack, someone with both high DEX and CON might well become effectively unhittable to anyone without a serious level advantage).

I'm definitely not trying to argue that Constitution is in all cases a more reasonable stat than Dexterity for AC. There's an argument for both. Honestly there's an argument for all the attributes except possibly Charisma feeding into your character's defense. That's clearly one of the places where simulationism runs into the wall of game design considerations.

I suppose what I was more trying to get at is questioning an assumption made by a lot of people in the RPG world, including myself in the past. I think a lot of us picture medieval combat as going down the way it does in samurai movies, where as soon as someone gets hit with a sword, a fountain of blood goes up and they drop to the floor. I suspect there was actually a lot more exchanging of blows. While a hand weapon is inevitably a lot more damaging than a bare hand, one thing you from hear all corners (whether it's historians, doctors, military etc.) is that it's actually quite a bit harder than people expect to deliver a fatal (or even debilitating) blow with one. Combine that with the observation that in modern sport combat it's rare for even a professional fighter to come out of an exchange untouched, and toughness as a component of AC makes sense to me.

I suppose what I'm really arguing here is less about game mechanics and more about the way people should imagine a fight in their games. Now that I come to think about it, from a mechanical standpoint my theory is probably more of a justification for the HP system than anything else. Constitution already affects that, which is good enough in most cases.  Folding it into AC makes the most sense in something like Index Card RPG, where you have a very limited HP (10 for the vast majority of characters), and Constitution doesn't otherwise improve it.

In my DtwenD system that I'm currently developing, there was an incident yesterday that emphasized some zombies using Endurance (the physical and mental fortitude stat) as the defensive stat die to roll.

If an attack roll is, more accurately, a "to-hurt" roll rather than an accuracy test, then I think it makes sense to incorporate CON into ACin lieu of HP. Then again, I would also incorporate the number you exceed your to-hurt roll as bonus skill-based damage as I currently do in DtwenD.

  I would argue in the context of being in shape and defense, that applies to hit points...as the bruise on the cheek instead of a fracture or KO is more akin to the fighter having the skill and experience to move just enough to have the blow slide off or to roll with it to a degree (this also leads us back to DX).   In the old school sense hit points are largely metaphysical instead of pure beef to absorb damage.  So in a sense constitution has always contributed to defense...the more of them you have the more you have to lose before you start taking serious bodily injury.