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BardicDuelist
2007-12-11, 11:26 PM
I was thinking, after reading GURPS, that d20 is sometimes considered a universal RP system. The more I weighed this, I came to this conclusion: It is not a universal SYSTEM, but a universal mechanic.

That being said, it is a mechanic I enjoy, although I like the GURPS rules a great deal. Does anyone know a HR or variant rule that uses d20 for GURPS?

I don't want to change the majority of the game, just make it so that normal rolls (except for damage rolls) use d20 instead of 3d6. My mathematical skills are not up to par when it comes to this kind of thing, and somehow I think that just "subtract two" from the result would be too simple.

Grynning
2007-12-11, 11:41 PM
Not as such, the two systems are a bit too different. However, Mutants and Masterminds second edition combines a really good point buy system with the d20 mechanic, and also eliminates all the other dice, so you only need a d20. They published a Universal (non-superhero focused) version of it under the name "True D20", IIRC.

Edit: "They" being Green Ronin Publishing, and it's just called "True 20"

BardicDuelist
2007-12-12, 12:11 AM
Theoretically, wouldn't replacing "roll 3d6" with "roll d20" and counting 1-3 as a critical success, 18-20 critical failure work?

Since 3/20 is 15%, and 2/15 is about 13%?

Note: 3d6 yields 15 possible numbers, not 18. 3 or 4 is critical success, 17 or 18 is critical failure.

Like I said, my math is rather weak in this area, so if there are any errors, please let me know.

Grynning
2007-12-12, 12:17 AM
????
Not sure if you meant to post this on a different thread...but you generate attributes and such with a point buy system in the M&M/True20 system, so there's no need for a 3d6 roll.

Gralamin
2007-12-12, 12:22 AM
Theoretically, wouldn't replacing "roll 3d6" with "roll d20" and counting 1-3 as a critical success, 18-20 critical failure work?

Since 3/20 is 15%, and 2/15 is about 13%?

Note: 3d6 yields 15 possible numbers, not 18. 3 or 4 is critical success, 17 or 18 is critical failure.

Like I said, my math is rather weak in this area, so if there are any errors, please let me know.

3d6 yields 16 possible numbers actually, so 2/16 is 12.5%.

BardicDuelist
2007-12-12, 12:56 AM
????
Not sure if you meant to post this on a different thread...but you generate attributes and such with a point buy system in the M&M/True20 system, so there's no need for a 3d6 roll.

I didn't mean to generate attributes. For "success rolls" in GURPS, one rolls 3d6. I was refering to GURPS, not M&M/True20.

Perhaps I need to clairfy. I want to roll a single die for success rolls, not three each time, and I am slightly sentimental to my d20. It is also more familiar to the players that I am teaching. Therefore, I was looking for a way to convert the standard 3d6 roll in GURPS to a d20 roll.

I like the M&M system, but I prefer how GURPS handles combat and want to play that system.


Gralamin: Ah, my mistake. Although 12.5 is still ABOUT 13, so the above should still work. Thank you.

Tengu
2007-12-12, 12:58 AM
3d6 yields 16 possible numbers actually, so 2/16 is 12.5%.

Except that there's a different change for each number. You are much more likely to roll, for example, 9 than 3.

Aquillion
2007-12-12, 01:14 AM
3d6 yields 16 possible numbers actually, so 2/16 is 12.5%.Wrong. And you call yourself a gamer. :smalltongue:

3d6 can come up in the following possible combinations, all equally likely:

111 112 113 114 115 116
121 122 123 124 125 126
131 132 133 134 135 136
141 142 143 144 145 146
151 152 153 154 155 156
161 162 163 164 165 166

...well, that's 1/6th of them. You get the idea. There are 6^3 combinations, or 216. Each one is equally likely -- but most of them are redundant, and different numbers have different numbers of (equally likely) combinations that can form them. For instance, there are more ways to get a 9 than a 3; since each of those ways is equally likely, the 9 is overwhelmingly more common.

Your chance of rolling a three on 3d6 is, in fact, only one in 6^3, or one in 216. Yes, you read that right. There are three ways to roll a four, so your chance of a critical failure under GURPS is 4/216, or 1/54. Ditto for your chance of rolling a critical success.

There isn't any way to emulate that on a single roll of a d20... obviously. You could come close by rerolling on a 1 and having a 1-7 on your reroll be a critical whatever, though... that'd give the chance of a critical on your d20 as (1/20) * (7/20) = 0.0175, vs 1/54 = ~0.0185 in GURPS.

But note that this wouldn't actually be adapting the system, since the 'normalizing' effect of 3d6 affects everything in Gurps, not just criticals. Rolls towards the middle of the range are more common; rolls towards the edges are less common. This means that even small bonuses have a much greater effect than you'd expect.

Pauwel
2007-12-12, 11:32 AM
May I ask why you want to convert the 3d6 mechanic into d20?

I mean, it has a wonderful bell curve that would be very annoying to copy with a d20, and it's not like six-sided dice are particularly common.

Do you just really like the icosahedric shape?

BardicDuelist
2007-12-12, 12:42 PM
Part of it is my strange fondness for the d20, yes. The other part is the simplicity of rolling one die. The final part is I have a lovely set of copper dice, and I want to use more than one of them.

I see that this isn't really that possible though. Thanks for the imput, as it was the bell curve that I was taking out of consideration (I know somthing was wrong, but I didn't know what).