Using Ability Checks in B/X

One gripe I have always had with D&D ability scores is - what are they for? Regardless of edition, they are only ever used for deriving other numbers. It would be much clearer if it were simply a -3/+3 stat since that is how it actually gets used. And yet, no one wants that. We all love our 3-18 rolls that we end up never using. It annoys me that such a prominent feature of PCs mechanically means so little.

The exception of course, is the ability check in TSR D&D, introduced in B/X and which achieved peak infamy with 2e non-weapon proficiencies. The mechanic where you actually get to roll against your ability score.

To begin with, let's acknowledge that there are many good old school reasons not to roll for ability checks in B/X:

• Most things that DMs in later editions require rolls for, shouldn't require a roll in the first place as long as the players can describe what they are doing properly - And the ability score should be factored in by the DM in those cases anyway.
• It encourages a binary "Success or Failure" approach to task resolution. 
• There are other mechanics already handling those events (mostly the X-in-6 roll).
• Ability scores should have low relevance to a character's capability compared to level.
• B/X assumes character competence and further down the road of ability checks lies the evil of looking at your character sheet to discover all the things you now can not do and sticking to what it says you can do. 

I basically agree with all these observations and would argue for them as reasons not to have a skill system. That notwithstanding, I want to have ability checks for for reasons of gaming aesthetics - It's one of the most prominent features of character creation and the character sheet. It should matter somehow, in and of itself and not just as a conversion to a different number.  There should be times where having rolled 18 Wisdom for my fighter actually matters mechanically.

Rolling against your ability score is one of the most obvious and intuitive rulings that the character sheet present to you. It belongs in the game somehow. Despite my support for all the observations in bullet points above, I believe there is a measured case for ability checks. Here's two ways I use them:

Make a CHA check to show the world that you are FABULOUS

Ability Check as Failover Mechanic

I call this the "Oh. Really?" roll. Use it in case of:

• Player failing to describe what they are doing properly.
• Players attempting poorly considered things, or failing to use the proper tools or preparation for the job.
• Players attempting foolhardy things that a competent person couldn't normally feel assured of accomplishing.

Basically, whatever could prompt the DM to say "Oh. Really?" can be immediately followed with a "make an ability check then." Make players aware of this guideline too - Ability checks probably means you could have played this one better. 

I believe this is also a nice bridge between those hardliners who insist in player competence over character competence and vice versa,

Make a WIS check to realise you need to level up before seeking revenge against Bargle

The second way I use it is

Ability Check as Cost Determination Mechanic

When players are attempting something that you have already determined will succeed, it can happen that circumstances suggest there could be a cost to it. 

For example, climbing a very difficult surface. The cost could be lost time (always a precious resource when you track time with the random encounter die), broken tools, exhaustion, injury, etc. Or the inverse - even faster than expected, went up real quiet, etc. 

For that, you roll an ability check, not to determine success or failure, but to have a "margin of success or cost." You then use the margin of the die roll against the ability score to inform your judgement of cost or additional success.

In this way, ability scores don't go down the road of gatekeeping who can and can't do tasks or creating binary outcomes from something that doesn't need binary outcomes, but they still inform you how a character interacts with the world based on those scores.

Modifiers are simple - Grant advantage if you have especially favorable circumstances (for example, attempting something especially relevant to your class or secondary skill) and disadvantage for especially hard circumstances. Keep reading to find out why this is mathematically pleasing in relation to the regular X-in-6 roll. 

Walking on the ceiling is a DEX check, right?

Using ability checks in place of X-in-6 rolls

As a last exercise, let's see if there are not other scenarios where an ability check can be used: Converting the X-in-6 roll.  

I use this basic formula for conversion from X-in-6 to to a d20 roll: (20/6)*X. This tells me the "Target Number" I am trying to roll at or under on a d20. With rounding, it goes like this:

• 1-in-6: 3 (3,33)
• 2-in-6: 7 (6,66)
• 3-in-6: 10 
• 4-in-6: 13 (13,33)
• 5-in-6: 17 (16,66)

This progression maps also very nicely to the bonuses given by ability scores, if we take 3-in-6 and 10 as our respective baselines. A 7 ability score gives a -1 ability modifier. 13 gives a +1 and 17 +2. 

Which means that ability score modifiers applied to an X-in-6 roll gives about the same benefit as the ability score itself when applied to a standard 1d20 roll under check. 

So for those 3-in-6 rolls where ability score modifiers apply, you can basically just make an ability check and get roughly the same outcome, albeit with more granularity. For the hard ones starting at 2-in-6, apply-3 to the ability check and -7 for 1-in-6 rolls. And vice versa, +3 for 4-in-6 and +7 for 5-in-6

However, there is another way to get much closer to those unrounded probabilities of the d6, using a modern mechanic instead of those static modifiers to the roll - Advantage. 

For 2-in-6, apply Disadvantage to the roll.

[dis-]advantage skews the average of about 3.4 so is actually closer to the unrounded d6 progression of  3.33. 

For those rare 1-in-6 roll, apply a further -3 to the d20 roll (the probabilities of advantage interact with the static modifier, hence hewing to -3 rather than -4).
For 4-in-6, grant Advantage and 5-in-6, an additional +3 to the roll.

I like advantage over static bonus here, because it means high ability scores retain a (very) small chance of failure that gets lost with static modifiers and overall it is more pleasing to roll differently for different circumstances than adding and subtracting, especially when the probability math of advantage is so close to the original X-in-6.

Looking over the X-in-6 rolls in B/X, let's see which ones allow modifiers from ability scores and can therefore be translated into an ability check:

• Climbing (the rules even recommend an ability check instead)
• Opening Stuck Doors. 2-in-6 -> STR check with disadvantage.

Ok, that is actually not a lot, although I do believe stuff like boarding vessels and hiding could receive bonuses from ability score modifiers. This exercise is perhaps more about examining the maths and demonstrating some equivalence between the ubiquitous X-in-6 roll and the ability check, showing that there can be a place for it in B/X.