Old-school dragon game systems often use roll-under mechanics for ability checks. While this seems simple, it’s often confusing and always clunky.

Roll-under

In a roll-under system, to make a dexterity check, for example, you roll 1d20. If you roll your dexterity score or less, you succeed. This means that a natural 20 is bad and a natural 1 is good, which is counterintuitive. It also means that any bonus to the roll needs to be subtracted from the die result or else added to the target number. So, for example, if you roll a 14 and have a 13 dexterity, you fail. But if you have a +1 on the roll for some circumstance, your roll is modified down to 13 or—if you prefer to think of it this way—the target is moved up to 14; and you succeed. Either way, it’s counterintuitive and therefore clunky.

Without the bonus, you had a 35% chance of failure—14 or higher on a d20. And you cheer a natural 1, which is weird.

The Fix

The fix is to simply say that your target is always 21, and you roll 1d20 and add your dexterity score (or other applicable ability) as a bonus, plus any modifier for circumstances. So now, you roll a 7 and add your 13 dexterity for 20—a failure. But if you get a +1 for circumstances, you’ve rolled a modified 21—a success.

Again, without the bonus, you had a 35% chance of failure—7 or lower on a d20—and now you don’t have to cheer a natural 1.

Of course, if you don’t care about preserving the math exactly, you can say the target is always 20, which feels more right. A score of 18 can still fail on a roll of 1.

Caveat

However, this confronts us with a pretty glaring issue. If you always need at least 21, and you’re rolling a d20, it’s always possible to succeed, even if you’re naturally clumsy but also drunk. Maybe the die should really be a d12, so that characters will a really low ability score will find it impossible.

That’s for you to decide. You may want certain ability checks to be impossible with a low ability score. Or you may want such tasks to be possible even for a drunken klutz.