Tuesday, January 29, 2013

The Dog Throw - The disruptive 2d10

The 2d10 can generate numbers in a range similar to 1d20, but without the linear distribution that characterizes 1d20. This is handy when you want to favor certain rolls over others. With a d20 a player is as equally prone to roll 1 than 5 than 15 or 20. So the odds for outstanding success and utter failure are the same. In life this is generally not the case. If a character is good at something he would expect to be good at it most of the time, not fumble it for no apparent reason. It makes me wonder how come characters don't end up neurotic or just completely mad.

Unfortunately the 2d10 has a drawback, each step isn't exactly separated by 5% chance of success or failure. This makes it complicated for GMs who are used to 5% steps to adjust probabilities.

The distribution for 2d10 compared to 1d20 looks like this:

Roll % 2d10 % 1d20
2 1 10
3 3 15
4 6 20
5 10 25
6 15 30
7 21 35
8 28 40
9 36 45
10 45 50
11 55 55
12 64 60
13 72 65
14 79 70
15 85 75
16 90 80
17 94 85
18 97 90
19 99 95
20 100 100

It has a very low probability of rolling low numbers, 6% of rolling 4 or less compared to 20% on the d20, and similarly for values 17 or higher. This makes it a terrible option if you play games that require to hit a certain high value like some 18 or 19 armor value. It will be very hard for players to roll that high.

On the other hand 2d10 favors skill and adds predictability to the game. Lets change the rules a little and say a hit is achieved on a roll of 12 or higher. So our character fails on a roll of 11 or less, which is 55% for both 2d10 and 1d20. If the character enjoys a +1 modifier to hit, then an 11 is required and the odds of failing are equal to rolling a 10 or less. This amounts to 45% on 2d10 and 50% on d20. A skilled character has less probability of failing with 2d10, but this early benefit is offset by a slower progression later on. While the d20 continues to gran 5% benefits on every plus after +5 the 2d10 begins to give less and less for each plus. Meaning it becomes harder to get good the better the character already is. Which is, ironically, may be seen as a good thing. I certainly believe greater progression should have considerably greater cost. Don't you think?

So 2d10 has some really good benefits, like a non linear distribution that gives some predictability to the rolls and makes higher modifiers have less benefit. On the other hand it makes rolling a 20 an event one in one hundred, so forget about using it in classic d20 systems. It would simply mean a complete rewrite of all the combat mechanics.

So, as a player or GM have you played games that use such a roll? As a designer have you considered it as part of your mechanics? Yes, no, why? I make strong use of it in the combat and skill mechanics and would like to know what others think of this die roll.

Image is a 3D printable item you can acquire here:

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