My recent meditations on roll under ability checks and rant against the D20 unified mechanic has generated a bit of commentary and further clarified my own thinking on the matter. This post is an addendum to my  Using Ability Checks in B/X  article, seeking to further explicate why the "Roll Under" ability check truly is the best of ability checks. Earlier today, during my delvings into the blogosphere, I came across this box from  Quarrel & Fable , a Fighting Fantasy spinoff: First thing that struck me was how similar it was to my proposed resolution for Ability Checks . And secondly, it combined those thoughts with my memories of the old Fighting Fantasy  [FF] gamebooks and set my mind spinning into that cross section and how much I always liked the elegant simplicity of the FF mechanic. The best part about doing a post involving Fighting Fantasy is the chance to showcase some of the brilliant art in the gamebooks Now, I've given reasons already in previous posts

It seemed revolutionary at the time. 3e came out and made a unified mechanic. Roll 1d20 against a target number to see if you succeed. In combat, AC is the target number. For everything else, it's a Difficulty Class [DC]. That's it.  Some of those DCs are calculated as a function of level, opposing ability score etc. But what we also got from this system was a way of ad hoc determining the difficulty of something and then simply saying "roll against that target number to succeed".  In its core form, this is wonderfully simple and intuitive. All you need to internalise is the size of the numbers on a d20 in relation to overall difficulty and then you can resolve basically anything with it. The part about size of numbers has proven to be a bit of an achilles heel for d20 over the years, but that is a different point I will address further below. No, the real point here is that there's an unintended side effect to DCs as a unified mechanic. There are other downsides