Every way to do Opposed Ability Checks

Modern game design tends to have very uniform and tightly-integrated resolution mechanics. But one of the things I’ve come to love about the rulings-not-rules OSR as well as the baroque mechanics of 1970s/80s (A)D&D is the way that trying to make sense of lots of overlapping and inconsistent rules really empowers the table to make the game their own. It forces GMs to really be the game designer, not just a referee—tabletop RPGs are best when GMs/players aren’t just passively consuming published material, but are making up their own stuff. To that end, these days I’m trying to collect as many resolution mechanic options as possible.

Ability checks—checking whether an action succeeds by rolling under a relevant ability score on a d20—is a resolution mechanic of long pedigree, and I use it all the time in my Labyrinth Lord game. Sometimes you need to do an opposed roll, though, and I’ve never seen anyone try to spell out all the ways to do that. Here’s a stab at it!

Opposed Ability Checks
In some cases, an ability check doesn’t represent a simple success or failure, but a contest between two or more people. This could represent an arm-wrestling competition, a singing or shoving contest, or a skillful card game. Opposed ability checks can be handled in several ways; the GM should determine which method is most appropriate to the situation.
Simple Comparison: The contestants compare the relevant ability scores and the highest one succeeds. No die roll is made. Any modifiers are applied to the ability score (for the purpose of this contest) before comparison. This is useful for contests which require a swift resolution or which innate ability is more important than chance.
Modified Ability Check: One contestant rolls a d20, and they receive a bonus or penalty on their ability check equal to 10 minus the opponent’s relevant ability score. In this case, rolling lower is better. For example, a wizard (Strength 8) is arm-wrestling a fighter (Strength 13) and gets a +2 bonus on arm-wrestling checks due to a spell he has devised. The number he must roll under to succeed is 8 + (10 – 13) + 2 = 7. This is most useful when a player character is vying against a single non-player character.
Highest Roll Wins: Each contestant rolls a d20 and adds their relevant ability score modifier and any other bonus. The contestant with the highest result wins. This is useful for contests of chance where a character’s innate abilities are a minor factor.
Highest Success Wins: Each contestant makes an ability check, and any who fail are eliminated. The remaining contestants apply any other circumstance bonuses or penalties to their roll, and compare results. The contestant with the highest modified roll wins. This is useful for contests that are a balance between chance and innate ability.
Highest Margin of Success Wins: Each contestant rolls d20, subtracts the result from their relevant ability score, and adds any other bonuses or penalties to the sum. The character with the highest modified score wins. For example, a thief with a Dexterity of 14 and a circumstance bonus of +2 rolls an 8. The thief’s margin of success is 14 – 8 + 2 = 9. In this case, rolling lower is better.

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