November 02, 2017

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Hi! Lovely article! I used to mostly play stormbringer, where we rolled two d20, one for "tens" and one for "ones" and make a percent from 0-99, 00 or 01 were critical fails, 98 or 99 were critical wins and would have DIRE consequences, like, lose a nose/amputated arm/permanent scar that affects your seduction skill forever changing your playstyle and ruining your life.... :(

but I digress. How do 2 d20 compare to your 3z8?

Hi Amit, I haven't played Stormbringer personally, but I believe it uses percentile dice, much like you described, but with 2d10s rather than 2d20s. Using percentile dice in this way also generates a uniform distribution similar to using 1d20, but spread out over 100 values rather than over 20 possible values. Basically using a system like this would fall somewhere between 1d20 and 1d20 with criticals, because the distribution is still uniform but criticals now have a 2% chance of happening rather than a 5% chance.

The reason 3z8 has a non uniform distribution is because since you sum the dice, there is more than one way to get a particular value. For example, you could get a 2 in three different ways; 1) rolling a 0 on the first dice and a 2 on the second dice, 2) rolling a 2 on the first dice and a 0 on the second dice, or 3) rolling 1 on both dice. This means it is more likely to happen that getting a critical miss of 0 because that can only happen 1 way; rolling 0 on both dice. Alternatively with the 2d10 percentile dice, there is only one way to get any number. For example, 17 can only happen by getting a 1 on the tens dice and a 7 on the ones dice.

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