I must create a system, or be enslaved by another man's. I will not reason and compare: my business is to create.

- William Blake

Monday, December 16, 2024

AD&D and ability checks - from d20 to d100

I have often wondered if people playing AD&D RAW use ability checks, and how often.

From a quick look at the rules, it would seem that if you don't, having Dexterity 7 and Dexterity 14 is identical. Same for Wisdom 8 and 14.

Is that part of the reason why Dexterity and Wisdom would become some of the most common saving throws in 2024 D&D? I'm not sure. 

Certainly rolling under Dexterity was used as a saving throw in some old school modules (to avoid falling into a trap, slipping, etc.).

For all other stats, however, there is some consequences to having a few extra points. The exact number are all over: Strength 18/33 gives you +1 to hit, +3 damage, +100 encumbrance, 50% chance of forcing doors and  20% of bending bars. Strength 18/53 will give you almost entirely different numbers.

[The table below if from 2e; the numbers are similar, but notice how they almost turned open doors into "roll under"].


I have a feeling that Gygax got enamored of the d100 some time between OD&D and AD&D. While AD&D uses multiple types of die, the d100 appears often, and it seems to be useful especially when the d20 is not granular enough.

I've tried streamlining theses numbers before, and maybe replacing some of them for ability checks.

One issue with using a d20 is that you lose the finer detail of chances that are lower than 5% (e.g., bend bars) of greater than 95% (e.g., system shock).

But this is not impossible to fix either.

Just revert to the d100 when (and only when) the d20 is not granular enough to give you chance of success/failure.

We could just use ability checks with a bonus/penalty; usually, usually from -4 to +4, but -10 for extreme tasks (e.g., bend bars). If something is impossible to roll on a d20 (e.g, you need to roll under 3 but you have a -4 penalty), we could give the PC an extra chance by rolling a d100 - your chances decrease by 1% instead of 5% or each point.

[Notice we sometimes say "roll under" when we really mean "roll equal or under"; for example, rolling under Dexterity 7 means you have to roll 7 or less on the d20].

For example, you'd need Strength 11 to even try to bend bars (similarly to AD&D). This requires rolling a 1, which means 5% chance. Strength 10 could reduce that to 4%, and Strength 6 to 1%. Strength 5 makes it impossible. Strength 19 gives you a 45% chance.

[I'd probably get rid of percentile Strength, BTW].

Same reasoning for system shock: say you roll with a +4 bonus. Constitution 3 gives you a 35% chance (exactly like AD&D), since you have to roll 7 or less. Constitution 15 gives you 95%. Since there is always a small chance of failure, Constitution 16 will give you 96%, Constitution 17 will give you 97%, up to 99% if you have 19.

This also works for thief skills, which follow a similar progression (move quickly to 95% then slow down). Say you need to roll under thief level +3 to hide or move quietly; this gives you 20% chance on level one, 95% on level 15, up to 99% on level 19. Of course, you could use ability checks instead (with a -10 penalty, for these are exceptional tasks, but adding thief level).

Another use for this: hitting negative AC. If your THAC0 is 20, you have 5% chance to hit AC 0. In AD&D, you ALSO have 5% chance to hit AC -2, but... wouldn't it be smoother if your just apply the negative AC to that 5% chance? So, AC -2 gives you 3% chance of success instead of 5%. AC -4 gives you 1%, AC -5 is impossible to hit. Easy!]

Anyway, I've been thinking about these concept of "stretching the d20" for years. Now that I think of it, it probably deserves a post of its own. But this "d20 to d100" stretching is enough for AD&D, I think.

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