How Do I Reverse Engineer Ability Checks from 3e to 2e?

This question relates to how the designers of 3rd edition might have come up with the roll-over Ability Check mechanics from the roll-under Ability Check mechanics of 2e. The problem I'm trying to solve is importing Ability Check mechanics from 3e to 2e. I thought by analyzing the transition from 2e to 3e, I might have a better understanding.

It's pretty common knowledge that you can reverse engineer to-hit rolls in 2e to be similar to those of 3e by:

Base Attack Bonus (3e) = 20 - THAC0 (2e)

AC (3e) = 20 - AC (2e)

But I'm at a loss as to how they might have determined the modifiers for Attributes to go from a roll-under 2e Ability Check to a roll-and-add 3e Ability Check.

I am lead to believe by various forum posts that, when 3e came out, "a lot of people were doing it that way already". If this is true, what were they basing these Ability Check rules on? Supposedly, there might have been articles in Dragon Magazine that included roll-and-add mechanics for Ability Checks. An issue number would be greatly appreciated if someone could provide one.

• Possibly related/relevant reading: Is UA's "Players Make All Rolls...?" In that it and answers dig into the meat of how to flip-flop roll-paradigms.
– nitsua60
Apr 18, 2016 at 14:33
• Could this have been a Skills and Powers thing? Apr 19, 2016 at 1:34
• Have not read skills and powers cover to cover but didnt see a roll over mechanic. NWP checks were still roll under. Apr 19, 2016 at 1:40

1 Answer

In 2e, each point of a character’s ability score gives a +5% to the chance of success. In 3e, every two points of a character’s ability score gives a +5% to the chance of success. Which is a fundamental difference.

If you want scores to count the way they do in 2e but you want roll-over like 3e, you’d want to use score - 10 as the modifier to the roll. (Assuming you use the same DCs as in 3e.)

(If I’m remembering 2e correctly. I don’t have my books handy to check.)