Ruin Explorer
Legend
As @Faolyn points out it's mostly when modifiers become involved that it gets a bit confusing, but usually this is due to the modifiers being badly done by a designer who didn't think about it hard enough in terms of expressing them, i.e. if it's roll-over, you get a +10% bonus to your roll, but to achieve approximately the same thing in roll under, just express it as a +10% bonus to your skill (i.e. the number you have to roll under, not to your roll). But sometimes we've seen clumsy stuff like -10% bonuses and so on and it's like, don't be dumb, think about your rules-design a bit harder!I've never understood the objections to roll-under.
If your odds of doing something are 17% then it's intuitive when rolling the d100 that up to 17 means you succeed and over 17 means you fail. It's more brain-work to flip it around and say you need to roll 83 or higher to succeed.
The only other issue with roll-under (which also applies to roll-over) is if you're switching back and forth in a single game, it can feel really bad, but relatively few games do that (AD&D did, but the design on D&D is pretty wildly primitive, arguably RC D&D, even perhaps BECMI had a more rational/modern design than even 2E).