The rule on modifiers applies only after you've determined a base roll. And to do that you must first resolve disadvantage (PHB page 173):
Use the higher of the two rolls if you have advantage, and use the lower roll if you have disadvantage.
Therefore, you always take the lower roll.
This is a direct inverse of this answer here for rolling a critical miss with Advantage.
It's pretty trivial if you allow occasional discarding of rolls/rerolls.
d2 (gonna need this later): if result is odd, read as a 1. If result is even, read as a 0.
d4: discard any results of 5 or 6.*
d5 (gonna need this, too): discard any result of 6.
d6: done.
d8: roll d2 and d4. Result is d2×4 + d4.
d10: roll d2 and d5. Result is d2×5 + d5.
d12: roll d2 and d6. Result is d2×6 + d6.
d20: roll d4 and d5. Result is (d4-1)×5 + d5. Or result is (d5-1)×4 + d4. It doesn't matter which, but just decide before seeing the results to avoid unconscious bias slipping in.
My kids have spelling tests every Friday, and every Friday at breakfast I take their list of 20 words and the 2d6 that are always in my pocket and quiz them in random order, using the 2d6 to simulate d20, d15, d12, d10, d8, d6, d5, d4, d3, and finally d2.
Once you get a little practice with it, it's second nature.
* — Okay, here's how I actually roll a d4, without having to discard any results. 1-4 stay the same. On a roll of five or six I look at the orientation of the die. If the numeral is closer to right-side up than upside-down, read the 5 as a 1 and the 6 as a 2. If it's more upside-down than right-side up, read the 5 as a 3 and the six as a 4. If it's pips (which mine actually are, little 8mm suckers) your read the 5 as a 1 if it looks more like a +
than an x
, and as a 3 if it looks more like a x
than a +
. The six you read as a 2 if the "lines" of three pips are closer to vertical than horizontal, as a 4 if the lines are closer to horizontal than vertical. In other words, ||
→ 2, =
→ 4.
Best Answer
"Natural" means an unmodified roll.
The number you see printed on the die when you just throw it. Not adding or subtracting bonuses, penalties or rerolling. Just the number you see.
Terms will differ in individual games and groups, but usually the total result (natural roll plus any modifiers) is just called your "roll," or we'll say "I got a 25." In some systems (sounds like you're reading a D&D book), your natural roll has implications regardless of the modifiers you can add to it, so it's important to have a way to talk about the difference.