Homebrew 2d10 vs 1d20

[NotAYakk]

2d10 is just 1d20 with some scaling.

Namely, DCs are ~0.5 lower, modifiers are SD(1d20)/SD(2d10) =~ sqrt(2) larger.

(VAR(1d20) is 399/12, VAR(2d10) = 2VAR(1d10) = 192/12, so VAR(1d20)/VAR(2d10) = 399/192 =~ 2. SD is the square root of variance.)

AnyDice

AnyDice is an advanced dice probability calculator, available online. It is created with roleplaying games in mind.

anydice.com

Select "Graph" "At least" to see how the two lines are basically on top of each other. I just moved the average, and scaled modifiers. (I threw in some x10 to make everything integers).

The place where they differ is in the "crit hit"/"crit miss" range; the outermost 5%, which in d20 is the "auto fail on 1" and "auto succeed on 20" area.

For 3d6, the average is the same, but the modifiers are effectively 2x larger than 1d20. So you can emulate the same thing by simply doubling all modifiers from a d20 system (and doubling DC distance from 10).

Ie, for 5e D&D, you'd add (attribute-10) to your d20 check, count proficiency as x2. DCs would double away from 10 (then subtract 1 for rounding purposes).

Strength 18, athletics(+3 prof) against DC 16.

With 3d6 they'd have a +7 against a DC 16, needing an 9+. The P(3d6>=9) is 74%.

Or 1d20+14 vs DC 21, needing a 7+, or 70% success chance.

3d6-2 strength no proficiency against DC 10 is a 37.5% chance.
Or 1d20-4 vs DC 9 needing a 13+, 40% success chance.

3d6 is "just" doubling modifiers and scaling DCs similarly.

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Multiple dice get interesting to me when you start playing with the dice themselves. Simply adding them up, the central region (where almost all rolls end up) is close enough to linear that we can ignore the missing twist (remember, we care about cumulative probability, not chance of an exact roll, and cumulative probability makes things more linear). Only the tails are significantly different, and those can be handled with a crit mechanic that gets the feel you want anyhow.

When you start playing with pairs of dice in your roll doing something (or similar) then rolling more than one die starts being more fun and interesting.

As an example, I have messed around with a percentile system. You roll 2d10, and you read the percentages both directions.

If you succeed with doubles that is a critical success. (so roll two 9s against a 45% difficulty)
If both directions beat your target number, that is a full success. (roll a 6 and a 7 against a 45% difficulty)
If one direction beats your target number, that is a partial success. (roll a 4 and a 7 against a 50% difficulty)
If both directions fail to beat your target number that is a (full) failure. (roll a 6 and a 7 against an 80% difficulty)
If you fail with doubles that is a critical failure. (So roll two 1s against a 27% difficulty).

Because there is very limited correlation between reading your 2d10 in each direction, this mechanic is quite similar to rolling twice. The chance of a crit is 10% of the base chance of a success/failure. If a failure is 0, partial is 1, and full is 2, and you ignore crits, the expected yield is 2 times the target percentage; at low percentages this is dominated by partial successes, at higher by full successes.

You can emulate this with a 1d20 type system by always rolling 2d20 and checking both against the target number, sort of like advantage or disadvantage. Doubles are less likely (5% times success or failure chance) than the above percentile system; if we add in "a critical is either doubles, or an extreme value (20 or 1) together with a full success or failure" you recover roughly the 10% of nominal success rate is crit rate.

You do need mechanics for partial/full successes for this to work. But I think it moves narrative forward better than a standard pass/fail system.

 

[MacDhomnuill]

Earthdawn turns this up to 11. A person's ability rating is the average roll for their dice, accounting for dice explosions. I.e. a step 13 is d12+d10, a step 20 is d20+d8+d6 (specific dice vary by edition, but statistics are statistics). Either way, the step becomes a reasonable expectation.

The game has a lot of modifiers so those step numbers go up and down constantly.

It is also, in many ways, 1e d&d. fighters are meat walls protecting fragile casters, who slowwwlly cast horrifyingly effective spells, assuming no one interrupts them.

This sounds fascinating now I gotta go pick up a PDF of earth dawn!

 

[aiouh]

Why not just let players choose based on narrative "positioning" and intent?

"Risky" 1d20
"Balanced" 2d10
"Conservative" 3d6

A GM might rule that a skill check must be rolled "Risky" because the player has to do something quickly.

A Player might elect to roll "Risky" by attempting a challenge in a more risky manner (higher highs, lower lows) even if more time is available.

Either way, changing all dice resolution mechanics over all challenges just changes the tone of the game. It's neither good or bad, I think.

 

[MacDhomnuill]

If you want a way to get a nice curve on a d20 while preserving a range of 1-20 try rolling 3d20 and take the middle # for a regular roll, high roll for advantage (assuming roll over) and low for disadvantage.

 

[AnotherGuy]

I've recently been reading up on older D&D and OSR, and systems there often use a 2d6 to determine things. The differences in weights compared to a flat roll, like a 1d12, for those rules makes bonuses and penalties feel different. So, while I'm not actually considering switching to 2d10 instead of 1d20, I wanted to discuss it and just ask what would happen if we did switch?

I suspect high AC would likely become an issue.
With this system I would likely incorporate Degrees of Success - particularly in combat.
One could also incorporate open ended doubles. i.e. a double 8, allows you another d10. If another 8 is thrown you can keep rolling a d10 (or you can say it is limited to only one d10).
Advantage would work better in my opinion - 3d10, select the highest 2 (or double if you are playing with the previous rule)