2022-10-03

On d6 Ability Checks

One of our Wandering DMs Patrons on our Discord server made a great observation: In AD&D Module WG4, The Forgotten Temple of Tharizdun, at one point Gygax calls for a roll-under-Dexterity-on-4d6 to avoid a net trap. More generally, it's been reported in play at tables run by Gygax and Kuntz that they would commonly call for checks in this fashion to roll 3d6 (you know, the same way you generate abilities in the first place for OD&D), 4d6, or 5d6 for tougher situations. 

Now most of us have probably at least heard of rolling d20 under an ability score as a classic check. Oddly, none of these ability-check methods were ever written into the core rulebooks for either OD&D or AD&D. (It does show up as one of the very last optional suggestions by Moldvay in his Basic D&D rules, 1981.) Was this one of those things that was so fundamental Gary overlooked ever writing it down? Or some other reason?

Anyway, the question was posed as to exactly what the success chances are with this method. Here's the result of a quick Monte Carlo program to estimate them (via C++ code on Github):

Chances to make a roll-under-ability-on-Nd6 check.

On the one hand, I personally like the theoretical elegance of a roll-under-ability-score (using some kind of dice) so very much that I often wish the entire system had been aligned to a roll-under methodology from the start, for every kind of check. 

On the other hand, a top complaint is that this makes ability scores too important in the game, whereas by the OD&D books you can legitimately play PCs with fairly unimpressive scores, because they make so little difference to the play of the game. Additionally, the chances for success are vanishingly small in many cases (less than 10% for scores 3-6 vs. 3d6; 3-8 vs. 4d6; and 3-12 vs. 5d6). As one of our top think-tankers wrote, "At which point why are you even rolling?".

A really short-and-sweet mechanic like this does whet my appetite occasionally. In this case the modifying number of d6s to adjust the difficulty seems neat and clean. Would you consider using a rule like this in your games? Or do you still use the classic roll-d20-under-ability idea?

31 comments:

  1. roll-d20-under-ability is my go to. Better math, easy, everyone knows about it.

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  2. I not only have considered it but have used that in my previous campaign as the way to do ability checks. I originally picked it up as a thing from the Hill Cantons, and decided to incorporate it.

    Although in general I also have kind of grown to dislike ability checks as players coming from newer editions just constantly, and I mean CONSTANTLY, ask you "ok what do I roll to do [blank]?" instead of, ya know, just saying "hey, I want to do [blank]." and moving on.

    In my next game my plan is to completely remove ability checks and either just give a flat x-in-6 chance to do something if a roll is needed, or just simply see if a character does or doesn't have a specific threshold to do the thing. For example "Well to lif this boulder you need at least 13 STR, as it's pretty hefty." If they have that, they can do it. If not, then they can not...unless of course they ask for help from a friend and combine their efforts!

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    1. And I also roll on single-d6 checks for many things (e.g., because that's a rule actually in the books for a lot of stuff).

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    2. @jenx After years of agonising about this, I've come to see your last paragraph as 'the way'. Keeps things loose, but allows everyone to understand the odds if a roll is needed.

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  3. I use a variety of ability checks (including d20 roll-under) for different purposes. For instance, I roll multiple d6 to determine the Strength of iron bars (e.g. 5d6). If your Strength score matches or exceeds the difficulty, you can bend them... This prevents a weaker character from getting lucky where a strong character failed (i.e., I roll for the iron bars once rather than have multiple players roll for their character).

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    1. That's an interesting point about the straight-cutoff method!

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  4. There's actually another game that uses 3d6, roll under, and add or remove dice for difficulty: The Fantasy Trip. Of course, attributes are supreme in that game.

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  5. I'm torn between roll under the ability score and roll over a target number. They both have their advantages and disadvantages. I think the most important thing for any campaign to do is to pick a system, keep it simple and stick with it. With that in mind, I really like this idea of changing difficulty with the number of dice, rather than a bunch of modifiers.
    Two changes I would make to the system presented though: First, I would add the character level to the ability score at the DM's discretion, though that could make some checks trivial at higher levels. So maybe level/3? Secondly, I would add two more difficulties; 4d6 drop the lowest and 5d6 drop the lowest. That gives a littles more granularity in the difficulty increases.

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    1. Interesting! In my own head I distinguish between stuff that's level-dependent (requires expertise: picking locks, making an attack, learning a spell) vs. raw ability (bending bars, jumping away from a trap). That former stuff gets rolled as d20 + level, while the latter is d6 + ability modifier for me, but I constantly consider other stuff.

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  6. There is sort of an ability check (called a saving throw) described in the Dig spell in the AD&D 1e PHB. If a character is within 1' of the pit, they have to roll under dexterity to avoid falling.

    When I DM I try to remind myself to resolve without dice first. If I feel a roll is needed I use the d20 roll for easier checks and a generalization of the open doors strength test for harder tests. Lately been toying with the idea of using a dice chain for those d6 checks, too. You know, d4 or d3 for easier, d8 and so on for harder. Haven't tried that yet, though.

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    1. Wow, that's great find in the Dig spell! (Also, I don't think I've ever seen that spell get used in my lifetime of gaming.) Just highlighted that in my book and I'll try to remember it in discussions like this in the future. What a weird place for that to get squirreled away.

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  7. I much prefer a simple 2 in 6 roll, adjusted by the ability modifier. By using 3d6 roll-under, note how your chances double when your ability score goes from 9 to 12, without any effect on the ability modifier.

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    1. I like that. It's far too elegant for my approach to 1E, but if I were to employ a mechanic that made sense given how ability score mods are distributed, that'd be it.

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    2. This is actually my current practice, too.

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  8. I switched to this for awhile from the normal d20 roll, but found the bell curve messed with the probability so eventually abandoned it. I see it as the bell curve is already incorporated into the pc at character creation, and then using the same method for ability checks is double dipping and just punishes players for bad luck on one roll at character creation

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    1. Yeah, we chatted about this some on our Discord server. Rolling bell-curve vs. another bell-curve gets multiplicatively weird. I argued in the past that what you really want (in theory) is the inverse of a bell-curve for these checks.

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  9. Generally, I hate roll-under in D&D and try to make everything work like Target-20 if possible.

    I would consider a variant with of this with a "drop the lowest" approach like various alternative ability score generation methods use to get a less drastic difficulty curve, a kind of back-door disadvantage mechanic (which I know you hate in turn, Delta).

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  10. Interesting to see the numbers on this. I don't like the flat d20 and would normally do 2d6 vs a target number with modifiers based on stats (-2 thru +2 as per CHA modifiers) and situational.

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  11. I have been doing this for just over a year in my home game - had done the math for 2, 3, 5, and 7 dice at a time to figure out difficulties/probabities - started as a way to replicate Thief skills for 0e, sans the Greyhawk Thief.

    It does make the ability score more important, but it doesn't hurt the game, I don't think - because most checks being between 2 and 5 dice mean that only truly exceptional stats make a difference: and it's fair that someone rolling a 16 or higher on 3d6 for stats should get *something* - but moreover, it's been fun turning it into a bit of a mini-game: picking a lock? 5d6, roll under. Have master picks? Roll 6d6, keep lowest 5. Stuff like that.

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  12. I use a system of roll ability x 5 or under on d% for routine checks (which is the same as roll ability or under on d20…) and modify the multiplier for difficulty of task. For example, roll ability x 4 or under on d% for a slightly more difficult task and for even more difficult tasks, reduce the multiplier to x3, x2 or even x1.

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  13. I’ve defaulted to ability checks (generally just on d20) many times for the same reason I think most do – expedience. For BX etc or 2E I think they make reasonable sense. For 1st Edition I don’t think they really suit the feel the game, for a number of reasons:

    • The abilities of characters are quite strictly delimited by class and level. There are things that anyone can do as a _player_ (solve a puzzle, plan an ambush, bribe a guard), and things that only certain characters can do (according to class and level), but not really anything that anyone can do by employing a generic mechanic. Obviously something like a saving throw is a generic mechanic, but it’s not quite the same thing as seeing if you can beat the hooded figure at chess by rolling under your INT.

    • It prioritises ability scores in the wrong way. Gygax is pretty clear that some of them should be high and giving bonuses, but the difference, say, between a DEX of 9 and a DEX of 14 is normally trivial – unless you start using dexterity to leap over chasms, in which case the difference becomes incredibly important.

    • It’s too simple a mechanic. I think the full and true 1E experience involves modular, idiosyncratic systems and processes, often badly explained, usually involving a table. If I want a unified or obvious mechanic I have 99% of other games to go to.

    At the end of the day, though, I do wonder why it wasn’t included in the core rules. I can only speculate that it was a question of value for money – if we’re going to charge people a small fortune for the PH and the DMG, we can’t just give them mechanics they could easily think up themselves.

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  14. I don't like ability checks per se as they emphasize ability scores, and I have my players generate them by 3d6 in order.


    I tend to use either a saving throw or a d6 roll instead. The system already has a built in method of determining success that increases as you level and that fits more with my preferred model that skill (level) trumps potential (stats).

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  15. "Oddly, none of these ability-check methods were ever written into the core rulebooks for either OD&D or AD&D."

    That's not correct. The illusionist's Phantasmal Killer spell (level 4; AD&D PH, p. 98) has a non-standard saving throw: "[t]he subject must roll three six-sided dice (3d6) and score a sum equal to or less than its intelligence ability score in order to disbelieve the apparition."

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    1. Good catch! Likewise, Lee above pointed to the Dig spell with its d20-vs-ability save. I guess I still want to say there's no general mechanic given outside these specific spell use-cases. Great finds, though.

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  16. If you roll 3d6 for generating stats and use 3d6 as a saving roll, that's the same statistically as rolling 4d6 drop lowest for generating stats and using the same for the saving roll, and the same as d% for stats and d% for saving roll. That is, if in whatever system you use, if you have a stat that only x% of the population have then your chance of failure is always x%. So it's quite a good system-neutral mechanic.

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  17. For White Star I'm using GM rolls 3d6 or 4d6, character succeeds if ability is equal or higher than roll. If not, PC rolls a Saving Throw to succeed anyway. Works very well IME.

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  18. I use this system in AD&D. It's only for exceptional circumstances and adding dice makes for an easier way of saying this is harder than giving + or - to a d20 roll. Since most ability scores are average anyway on a 3d6 this means a success is likely for most characters (with increasing levels of difficulty) - not a math analyst myselt but I'm guessing better chances for the 9-12 range than a straight D20.

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  19. Yeah, I'm certainly comfortable with OD&D-style roll-d6-low for most miscellaneous things not defined in combat or saving-throws tables, and the +/-1 or 2 ability modifier (similar range as in my OED) feels like a good fit for a d6.

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