What if CHA was the only spellcasting ability score?

[Greywander]

Something I've wanted to do for a while was find a way to make every mental stat do something for casters, allowing casters to specialize according to which stats they pumped. Kind of like how martials kinda sorta in theory can specialize through their physical stats: STR for melee damage, DEX for ranged damage, and CON for defense. In practice, almost every martial pumps one of STR or DEX, dumps the other, and then puts whatever is left into CON. But that's a separate issue for another time; right now I'm looking at casters and mental stats.

My approach to this issue starts by thinking about what kind of player would choose to play a caster that uses a particular stat, and what sort of gameplay they're looking for.

For INT, as far as I can tell, the kind of people that tend to be drawn to INT characters in general and INT casters (namely, wizards) in particular are those looking for something like a Batman wizard: someone who has a spell or item or ability ready for any situation they might encounter. Often this means a focus on utility spells, but it doesn't need to. So for what INT does for spellcasting, what I'm thinking is that INT would play a major role in determining how many spells you know or can prepare. Exact numbers are hard to discuss without a specific implementation, but we could perhaps imagine that someone who pumps INT would have about twice as many spells as someone who dumps INT.

WIS was a much tougher one, but I think I've found something that I like that fits well. WIS casters seem to be more about support, and that often means concentration. One of the issues that currently exists is that almost every caster has decent CON in order to help with concentration checks. This makes true glass cannon wizards a rarity, and gives almost every caster the physique of a marathon runner. Moving concentration checks to WIS makes CON much less important for casters. I'm not sure this would be sufficient by itself to make WIS worthwhile; I worry it would be like CON is for martials, where no one dumps it but no one bothers to pump it, either. So maybe WIS could also help you to concentrate on multiple spells. As a rough example, say you have a number of concentration points equal to prof. bonus + WIS mod, and a spell requires concentration points equal to its level or spell point cost to concentrate on it (or if you don't have enough, you can concentrate on only that one spell).

If you read the title of the thread, you know what I'm about to suggest for CHA. But it wasn't actually my first idea. The last time I considered this idea, my thought was that CHA could give you an extra spell slot whose level is equal to your CHA mod, representing having more power to draw on. But at that time I was still thinking that any mental stat could be your spellcasting ability score. But for some reason I don't really like the idea that your spellcasting ability can be different for different classes. Weapons all use predetermined stats (STR for melee, DEX for ranged), but for spellcasting it depends entirely on which caster class you took. So what if the only thing CHA did (for casters) was be the only spellcasting stat for all casters? Being a pansy doormat means your spells are about as powerful as a wet noodle, while someone bursting with confidence projects their force of will through their spells.

My worry is that anyone who plays a spellcaster will end up maxing CHA and then deciding between INT and WIS, similar to martials pumping STR or DEX and only taking CON as an afterthought. But I think it could actually work, and it fits with the archetypes for players who tend to go for certain mental stats. Utility and support spells generally don't care about spell attack or spell save DC. People have dumped INT on a wizard before, and it still works surprisingly well. You just have to focus on spells that don't require attack rolls or saving throws, and it turns out there's a lot of spells that applies to. So if CHA was the universal spellcasting ability, then there are still a lot of spells that could be quite effective without pumping CHA.

I think this also fits the archetype for people who tend to be drawn to CHA characters. CHA is all about manipulation, so it's fitting for it to make your spells that directly affect other people more effective. It's not just Fireball, but also Suggestion, or Hypnotic Pattern. CHA casters are the ones who want to be directly influencing other people, not passively supporting their allies, or busting out a utility spell to trivialize a puzzle.

What do you think?

[Maat Mons]

If you don’t like how every martial picks either Str or Dex, you could do what a lot of games do and have accuracy and power key off of different ability scores. For example, make everyone use Dex for attack rolls and Str for damage rolls. Nobody gets to use Str to hit or add Dex to damage. Of course, if you’re reimagining the ability scores, you probably want to address the other idiosyncrasies too. For example, physique is split into two different stats, Str for offensive physique, and Con for defensive physique. Either those should be merged into one, or Dexterity should be split into accuracy and avoidance, for attack rolls and AC, respectively.

I never really felt D&D got its own casting stats right. Clerics should use Cha, and so should all deity-themed casters. The entire reason a deity invests power into them is so they can go and proselytize to the masses, bringing more worshipers to the deity. Wisdom should be reserved for nature-themed casters. All arcane casters should use Int, even Sorcerers. Arcane magic is arcane, a synonym for complex and difficult to understand.

But I do support the idea of making all casting key off of a single ability score. I’d just make it a new ability score that doesn’t do anything else. If someone wants to build a spellcaster, they shouldn’t be pigeon-holed into being charming. They also shouldn’t be pigeon-holed into being brainiacs or … whatever you call people with high Wisdom scores. Wise guys? Yeah, that sounds right.

[LibraryOgre]

Something I played with a long time ago, for different editions, was to key different parts of spellcasting off different attributes.

Intelligence was maximum spell level; how complicated are the things you can understand. Wisdom was bonus spells; what is your mental endurance and willpower like. Charisma was spell DC; how powerful is your mind.

It somewhat goes back to Shadowrun, and pairing mental attributes with physical attributes... Int is mental Dex, Wisdom is mental Con, and Charisma is mental Strength.

I find it helps mitigate the power of casters by making them MAD.

(I went even further into MAD for this thing for 1e, where the distinction between spellcasters was mostly erased; good for a game without priests).

[Loek]

But I do support the idea of making all casting key off of a single ability score. I’d just make it a new ability score that doesn’t do anything else. If someone wants to build a spellcaster, they shouldn’t be pigeon-holed into being charming. They also shouldn’t be pigeon-holed into being brainiacs or … whatever you call people with high Wisdom scores. Wise guys? Yeah, that sounds right.

How about making the stat a choice for the caster (So probably the highest between INT/WIS/CHA), therefor not pigeon-holing anyone, but also not breaking stats in general by adding yet another one to focus on.

Instead, all 3 stats would have some aspect of casting associated with them. (Much like the physical stats all have their own thing, along side STR/DEX's ability to determine to hit)

(For a quick example)
INT for spells known
WIS for concentration
CHA for bonus slot

And your chosen stat does spell DC's and other normal spellcaster modifier things.

Alternatively, if you want to fully move away from a "casting stat", instead do: DC, to hit and concentration for the 3 stats, meaning you don't have to change the system at all, except look at different stats then normal.

[GalacticAxekick]

I never really felt D&D got its own casting stats right. Clerics should use Cha, and so should all deity-themed casters. The entire reason a deity invests power into them is so they can go and proselytize to the masses, bringing more worshipers to the deity.

Not at all! Proselytizing is a very new idea, and the majority of world religions (past and present) have no concept of it.

Proselytizing religion aims to describe the The Law of the Universe, the same way that science is meant to describe the law of the universe. No matter where you are, the truth is the same, so no matter where you are, you should believe the same thing.

Non-proselytizing religion aims to describe The Law of the Land, the same way that politics describe the law of the land. Greeks have no incentive to proselytize Egyptians, because Greeks have their rules and Egyptians have their own. On the contrary, their incentive was to worship harder amongst themselves to make their worshippers more powerful than rival worshippers.

D&D Clerics reflect this. While one Cleric might be aiming to spread the news of her god(s) so that her god will have more worshippers or so that more people will benefit from her god's care, another Cleric might simply be an agent of his god who does his god's work: a warrior who fights on his god's behalf, a prophet who shares his gods' wisdom or will with his people, or an exorcist who casts out otherworldly or foreign spirits. He could be socially inept and do all of these things very well!

All arcane casters should use Int, even Sorcerers. Arcane magic is arcane, a synonym for complex and difficult to understand.

Wizards rely on Intelligence because arcane magic is complex and they seek to understand it anyway. Sorcerers don't rely on Intelligence because Sorcerers don't understand arcane magic. It's an innate part of them.

It's like how it takes a very intelligent engineer or scientist to build a flying machine, or produce pharmaceuticals, or perform complex surgeries. The wizard is a magical scientist. But it takes no intelligence for a bird to grow wings, or for a snake to produce venom, or for a caterpillar to metamorphose. The sorcerer is a magical creature.

The sorcerer's innate abilities are not fueled by physical strength, but by strength of will. Charisma represents raw confidence (whether that's telling a lie with a straight face, or willing yourself to resist being banished by a spell), and so Charisma represents sorcerous casting.

[MrStabby]

One thing I am trying for a project I am working on is making spell casting ability not depend on the class but what the spell does.

So a healing spell would use the same casting stat irrespective of I it were cast by a cleric, a sorcerer or a wizard. An illusion or enchantment spell the same, likewise a summoning spell.

This helps step away from the casters able to do everything on their spell list equally well and is intended to to enforce some some natural strengths and weaknesses.

[Maat Mons]

With regard to Cleric, you say “He could be socially inept and do all of these things very well!” But that works much better as an argument against Sorcerer using Charisma. There’s no reason being strong-willed should make you good at interacting with people. Far from it, strong-willed people tend to be incredibly difficult to get along with. Good social interaction requires empathy, the ability to put yourself in another’s shoes. Even if you’re trying to manipulate someone to your own ends, being able to understand how they think will help you craft your words to better target them. Willful people don’t care what anyone else thinks. The idea of trying see things from another perspective is actively offensive to them.

The idea of Charisma representing force of will was just thrown on as a poor justification for Charisma being the Sorcerer casting stat anyway. If you look at the history of the game, this idea first shows up in 3rd edition, the same edition Sorcerer first shows up in. Before that, Charisma was strictly a social thing. Bard and Wizard were the only arcane casters, and they both used Intelligence for spells. Because being intelligent is what helps you deal with arcane things. Then along comes 3e with Sorcerer. Sorcerer, at the time, was literally just Wizard without the dumb parts of Jack Vance’s writings. Same spell list. No different class features, except that Wizard got metamagic-related benefits, and Sorcerer was worse at using metamagic than any other class except Bard. So the designers said to themselves “We need a justification for why Sorcerers depart from traditional Vancian mechanics.” Meanwhile, they’d also bee lamenting that Carisma had no use besides social skills. So they made up the idea that Charisma reflects force of will, and drew some very tenuous connections. Bada bing, bada boom, Sorcerers have their own casting stat. And Bards get to share it as an afterthought.

But if you’re just going to add “governs strength of personality” onto an ability score, it would have been more fitting to put it on Wisdom, due to it’s deep history of being used to resist mind-affecting attacks. And Wisdom also has the role of not letting people talk you into things, which is something force of personality would help with. Charisma is actually the worst possible choice, because likability is the one thing that’s anti-correlated with stubbornness in real life.

Even if you accept that being a Sorcerer removes the need to understand magic, there’s no reason, in lore, that it should add new requirements. Being innately tied to magic shouldn’t make it harder to use. If Sorcerers require great force of will to cast spells, so should everyone else. It would make far more sense if both Sorcerers and Wizards need Charisma to cast spells, but Wizards need Intelligence on top of that. Then, it would be the case that Sorcerers have an easier time casting spells, which could be justified by their innate connection to magic. This could also help compensate for the fact that spontaneous casters are stuck with the same spells all the time, but prepared casters are allowed to switch every day.

[GalacticAxekick]

With regard to Cleric, you say “He could be socially inept and do all of these things very well!” But that works much better as an argument against Sorcerer using Charisma. There’s no reason being strong-willed should make you good at interacting with people. Far from it, strong-willed people tend to be incredibly difficult to get along with. [...] Charisma is actually the worst possible choice, because likability is the one thing that’s anti-correlated with stubbornness in real life.

I have... exactly the opposite experience. I'd say being strong-willed (speaking with conviction, refusing to compromise on your boundaries, enduring pain or difficulty to do or get what you want) makes you much better at interacting with people. And I'd say being weak-willed (speaking with uncertainty, caving to the demands of others, giving up on what you want to do or get when it is difficult or painful) is repulsive. Some of my closest friends are people with convictions they absolutely won't compromise on, and people who will speak uncomfortable truths (or at least uncomfortable opinions) without hesitation.

I don't expect this information to change your mind. It's my experience vs yours, after all. I'm just putting my perspective out there so you can see where I'm coming from. If we called the Charisma stat Confidence, I would 100% expect it to govern skills like persuasion/deception/intimidation and a Sorcerer's willpower-based casting alike.

Good social interaction requires empathy, the ability to put yourself in another’s shoes. Even if you’re trying to manipulate someone to your own ends, being able to understand how they think will help you craft your words to better target them.

Absolutely! And this is where Wisdom (Insight) comes in. Wisdom (Insight) tells you how to approach a social interaction tactically, while Charisma (whatever) decides how you navigate the interaction technically. Wisdom (Insight) tells you "this guy has insecurities I can threaten and a dream I can dangle in front of him". Charisma (whatever) decided how legitimate your threat and your promise sound. There's a great video on this.

Willful people don’t care what anyone else thinks. The idea of trying see things from another perspective is actively offensive to them.

I don't think that's true at all. I think you're conflating confidence and narcissism.

But if you’re just going to add “governs strength of personality” onto an ability score, it would have been more fitting to put it on Wisdom, due to it’s deep history of being used to resist mind-affecting attacks. And Wisdom also has the role of not letting people talk you into things, which is something force of personality would help with.

I agree, but I take it in the opposite direction. I think "not letting people talk you into things" and "resisting mind affecting attacks" should be Charisma saves instead of Wisdom saves.

Even if you accept that being a Sorcerer removes the need to understand magic, there’s no reason, in lore, that it should add new requirements. Being innately tied to magic shouldn’t make it harder to use. If Sorcerers require great force of will to cast spells, so should everyone else. It would make far more sense if both Sorcerers and Wizards need Charisma to cast spells, but Wizards need Intelligence on top of that.

Not really. Going back to the scientist/engineer vs creature analogy:

When a bird flies, it has to flap its wings. It's making a physical effort. Likewise, when a Sorcerer casts, he has to exercise his will. He's making a metaphysical effort. When an engineer flies, he makes no effort. Designing the aircraft was all the effort. Likewise, when a Wizard casts, she doesn't exercise her will. Learning and preparing the spell was all the work.

Or using a different analogy:

Suppose I ask two people to open a locked chest: a locksmith and a strongman. The locksmith opens the chest effortlessly using his knowledge of how locks work and his tools. The strongman opens the chest without knowledge of how the lock works by smashing or prying it open. The locksmith is to the Wizard what the strongman is to the Sorcerer. One takes Intelligence. The other takes (will)Power.

This could also help compensate for the fact that spontaneous casters are stuck with the same spells all the time, but prepared casters are allowed to switch every day.

I think the simple specialist vs generalist divide is enough. Sorcerers have a narrower spell list, they are better at casting all of those spells thanks to Metamagic.

What I would change is that this divide isn't dramatic enough. I think each sorcerer subclass should have its own spell list that only includes the theme of that subclass, and I think that sorcerers should gain more powerful and/or cheaper Metamagic effects to make them the definitive masters of whatever they were literally born to do.

[Maat Mons]

The bird versus aircraft analogy would better suit a comparison between spellcasting and use of a magic item. Flying an aircraft is like wearing Wings of Flying. You don’t need to be born with wings / a magical gift, and you don’t need to study aircraft design / wizardry. Sorcerers and Wizards both expend metaphysical effort to cast spells, because that’s what spell slots represent. If preparing a spell were all the work, you’d be able to cast it however many times you wanted for the rest of the day after preparing it. But you can’t, because you need to expend spell slots / exercise your will to do the actual casting. The preparation process doesn’t replace anything, it’s an additional step Wizards must take that Sorcerers can skip.

For the lock analogy, I’d say both the Sorcerer and Wizard would equate to picking the lock. The Sorcerer would be a lockpicking savant, taking to the process like a fish to water. The Wizard, on the other hand, is someone who had to work very hard to get the hang of lockpicking. It’s not really a different approach, it’s a greater inborn aptitude for the same approach. This is why Wizards are traditionally depicted as old. They have to work hard for a long time just to reach the point Sorcerers start at.

I don’t think Metamagic is enough to make up for not being able to change spells. I also don’t think it’s enough to make up for having half as many spells known as a Wizard has spells prepared (except Tasha’s archetypes). It’s also a pretty jarring reversal of earlier editions roles of Wizard and the metamagic master, and Sorcerer as being nearly unable to effectively use metamagic. Specialist versus generalist could be a division of roles, but the specialist would need be better at their thing by a wide enough margins to justify the loss of versatility, and I really don’t feel that’s where we’re at.

What you’re describing with premade spell lists is essentially 3rd edition’s Beguiler and Dread Necromancer (and Warmage, but we don’t speak of that class). Beguiler and Dread Necromancer worked as specialists because they got all the spells in their specialty. That was 104 spells for the Beguiler, and 82 for the Dread Necromancer. Compared to the closest generalist class (Sorcerer, as it happens), who got 34 spells known, that’s well over double. So if Wizard is the generalist, and Sorcerer is the specialist, it should be sorcerer that has twice as many spells available as the Wizard, not the other way around.

On the plus side, the Beguiler / Dread Necromancer model does make it impossible to screw yourself over in the long term. The fixed spells known formed a good, functional list, that was useful across a decent breadth of situations. Unlike Sorcerer where, if you realize you hate all your spells known, mostly just leaves you to suffer. Cleric, Druid, and Wizard also make it impossible to screw yourself in the long term. If you realize you hate all your spells prepared, tomorrow is a new day.

[GalacticAxekick]

The bird versus aircraft analogy would better suit a comparison between spellcasting and use of a magic item. Flying an aircraft is like wearing Wings of Flying. You don’t need to be born with wings / a magical gift, and you don’t need to study aircraft design / wizardry.

Agreed! That analogy was poor for the wizard/sorcerer divide.

Sorcerers and Wizards both expend metaphysical effort to cast spells, because that’s what spell slots represent. If preparing a spell were all the work, you’d be able to cast it however many times you wanted for the rest of the day after preparing it. But you can’t, because you need to expend spell slots / exercise your will to do the actual casting. The preparation process doesn’t replace anything, it’s an additional step Wizards must take that Sorcerers can skip.

Wizards gain rituals (spells that can be cast an unlimited number of times), Arcane Recovery (the power to regain lost spell slots through study), and Spell Mastery/Signature Spells (the power to cast simple spells they have studied hard for free) to represent the fact casting spells is not a serious effort. All of this points in the direction that Wizards can cast spells effortlessly, and that all it takes to regain spell slots is remembering how their spells work.

The only reason why Wizards are limited by spell slots is balance. It's a thematic sacrifice for mechanical purposes.

For the lock analogy, I’d say both the Sorcerer and Wizard would equate to picking the lock. The Sorcerer would be a lockpicking savant, taking to the process like a fish to water. The Wizard, on the other hand, is someone who had to work very hard to get the hang of lockpicking. It’s not really a different approach, it’s a greater inborn aptitude for the same approach. This is why Wizards are traditionally depicted as old. They have to work hard for a long time just to reach the point Sorcerers start at.

And I'd say that the Sorcerer are Wizard are taking fundamentally different approaches. The Wizard is trying to understand the lock and reverse whatever mechanism makes it work. The Sorcerer simply does not understand the lock, but has the natural ability to bypass it through effort.

I don’t think Metamagic is enough to make up for not being able to change spells. I also don’t think it’s enough to make up for having half as many spells known as a Wizard has spells prepared (except Tasha’s archetypes). [...] Specialist versus generalist could be a division of roles, but the specialist would need be better at their thing by a wide enough margins to justify the loss of versatility, and I really don’t feel that’s where we’re at.

I agree. That's not where we're at. Like I said, I think the Sorcerer should have cheaper and more powerful Metamagic.

It’s also a pretty jarring reversal of earlier editions roles of Wizard and the metamagic master, and Sorcerer as being nearly unable to effectively use metamagic.

I don't care at all about tradition.

What you’re describing with premade spell lists is essentially 3rd edition’s Beguiler and Dread Necromancer (and Warmage, but we don’t speak of that class). Beguiler and Dread Necromancer worked as specialists because they got all the spells in their specialty. That was 104 spells for the Beguiler, and 82 for the Dread Necromancer. Compared to the closest generalist class (Sorcerer, as it happens), who got 34 spells known, that’s well over double. So if Wizard is the generalist, and Sorcerer is the specialist, it should be sorcerer that has twice as many spells available as the Wizard, not the other way around.

Sure! The Sorcerer can learn more spells from a narrower range defined by its subclass. Maybe even just learn the whole list. The Wizard can learn fewer spells from a broader range defined by its class. That's fine.

On the plus side, the Beguiler / Dread Necromancer model does make it impossible to screw yourself over in the long term. The fixed spells known formed a good, functional list, that was useful across a decent breadth of situations. Unlike Sorcerer where, if you realize you hate all your spells known, mostly just leaves you to suffer. Cleric, Druid, and Wizard also make it impossible to screw yourself in the long term. If you realize you hate all your spells prepared, tomorrow is a new day.

Absolutely!

[Greywander]

I never really felt D&D got its own casting stats right. Clerics should use Cha, and so should all deity-themed casters. The entire reason a deity invests power into them is so they can go and proselytize to the masses, bringing more worshipers to the deity. Wisdom should be reserved for nature-themed casters. All arcane casters should use Int, even Sorcerers. Arcane magic is arcane, a synonym for complex and difficult to understand.

On the contrary, I think the casting stats fit most classes pretty well. Wisdom, on a metaphysical level, is about being in harmony with... well, whatever it is you're in harmony with. Could be nature, could be the will of your deity. A cleric with a high Wisdom is more in tune with their deity, and naturally aligns with that deity's will. This allows the cleric to act as a stronger conduit of that deity's power with less "waste".

As for sorcerers, in the thing I'm working on they're not arcane casters at all. Rather, sorcery is a form of innate or "natural" magic. Basically, any time a monster uses a supernatural ability, that's sorcery. Now, that said, in a lot of cases like that, I might be inclined to use CON instead of CHA as the casting stat. Especially for things like racial spells or other innate abilities. So I might need to figure out how and when to make that distinction.

How about making the stat a choice for the caster (So probably the highest between INT/WIS/CHA),

I'm specifically trying to avoid this, though. When you think about it, it's just strange that apparently any mental stat can be used as a spellcasting stat. That kind of fuzziness doesn't exist when it comes to weapons; it's usually very clear which stat a weapon should be using (though there are some disagreements, e.g. STR bows).

I think this could actually work out pretty well, though. Dumping INT on a wizard is a meme build, but it's an effective one, particularly because there are a lot of spells that don't care about your stats. So if CHA is what influences your spell attack and spell save DC, you can still play an effective caster even while dumping CHA. If the other mental stats are also giving something of value when it comes to spellcasting, then it becomes a matter of how you want to specialize. You might not be able to get something like Hypnotic Pattern to stick worth a crap, but if as a trade off you can concentrate on two or three spells, that's a huge deal.

One thing I am trying for a project I am working on is making spell casting ability not depend on the class but what the spell does.

So a healing spell would use the same casting stat irrespective of I it were cast by a cleric, a sorcerer or a wizard. An illusion or enchantment spell the same, likewise a summoning spell.

This helps step away from the casters able to do everything on their spell list equally well and is intended to to enforce some some natural strengths and weaknesses.

I've also considered the idea of casting stat being based on the school of magic. I think it could work well in a different context, but I don't think it's a good fit for the particular project I have in mind.

[Maat Mons]

So, do you think Paladin should be a Wis-based caster? Since a high Wis would allow the Paladin to be more in tune with the deity they serve? Because if a Cleric can't power their spells by strength of conviction, I don't see why a Paladin would be able to. I've always viewed those two classes as birds of a feather.

[Greywander]

Yeah, I think WIS generally makes more sense for paladins than CHA, but paladins are also a bit different than clerics in that they don't necessarily need to draw their power from a deity. I think you could also have a cleric who draws power from the strength of their conviction rather than from a deity. Honestly, I think we could let any caster freely pick which stat to use for casting, and a lot of them would make sense. A legalistic, orthodox cleric draws power from their knowledge of their scriptures and religious law, hence INT casting, for example.

But what I want to do is, instead of just picking one stat and not getting any benefit from any other stats, I want to make each stat give a different benefit. Then it's not so much a matter of which stat powers your spellcasting, but rather which stat bonuses are more important to the style of spellcaster you're wanting to play.

[Maat Mons]

Small nitpick, I’m 90% sure Xanathar’s says a Cleric doesn’t need to worship a deity.



The abundant room for debate on which stat should govern any given casters spellcasting is part of why I advocate for a separate “Magic” ability score.

Disagreement over exactly what roll should fall under which mental stat, and what each mental stat should represent, is why I advocate for merging all the mental stats (Int, Wis, and Cha) into one.



If you want, you could harken back to previous editions, and make Dexterity govern attack rolls for spells. No particular reason shooting someone with a ray that comes out of your finger needs to follow different rules from shooting someone with a hand crossbow bolt. You can even make the little pistol-shooting gesture when you can your spell.

If you really want to harken back to previous editions, you could put a hard cap on maximum spell level available based on an ability. It used to be max spell level = casting stat - 10, which was one of the few occasions where an odd-numbered ability score mattered.

3rd edition also gave bonus spell slots for high ability scores.