I've done quite a bit of work in exploring dice mechanics in various games. Here, I'm writing down a different mechanic for a role-playing game that is different from what I've seen. If someone has already developed this idea, then please let me know. The MOD7 part of the name comes from the mathematical operation called modulus. The modulus is the remainder after you divide two numbers. For example, 16 divided by 5 is 3 with a remainder of 1. Another way of saying it is the remainder of 16 divided by 5 is 1. Still further, the modulus 5 of 16 is 1. All three are different ways of saying the same thing. Modulus is a term used when you only care about finding the remainder.

So the MOD7 part of the name means we are looking for the remainder of something after dividing it by 7.

The D6 part of the name means that we are going to roll six-sided dice.

Put them together and you have rolling a number of six-sided dice, and taking the modulus 7 of the result.

Basic Mechanic:

If Attribute + Skill + D6 MOD7 roll is equal to or above a target number, the action has been performed successfully.

D6 MOD7 Roll:

There are two kinds of D6 MOD7 roll, a major and a minor roll. A Major roll is the modulus 7 of the total of rolling three regular six-sided dice. In other words, roll three regular dice, find the total, and figure the remainder after dividing that total by 7. A Minor roll uses two regular dice instead of three. Most players will only use a major D6 MOD7 roll.

Too Much Math?:

At first, it seems like a lot of work to get a number. It becomes automatic after a few rolls. However, if you want a table to make things easier at first, here it is:

3 -- 3
4 -- 4
5 -- 5
6 -- 6
7 -- 0
8 -- 1
9 -- 2
10 - 3
11 - 4
12 - 5
13 - 6
14 - 0
15 - 1
16 - 2
17 - 3
18 - 4

Why bother?

The reasons behind this mechanic are:

  1. the chance to roll a zero
  2. a way to give players a small advantage that is fairly invisible
  3. a more equal chance of rolling any number

Rolling Zero
The chance to roll a zero is to add some variety. Rolling a zero is not an automatic failure. It merely represents those times that 'luck' isn't on your side. The good news is that players have a better chance of rolling anything rather than a zero. Folks that use a minor roll (usually controlled by the game master) have a better chance to roll a zero.

Invisible Advantage
The major roll has more chance of success than a minor roll, but it isn't apparent from anything other than the number of dice thrown. The advantage isn't huge, but it's enough to make a difference. In the math section, you can see the breakdown of exactly how much of an advantage the major roll provides.

Even Distribution of Rolls
Rolling three dice and using the total alone tends to give you average rolls. In other words, you'll roll average rolls more often than small or large rolls. This is seen by many experienced rpg players as giving an advantage to players with powerful characters. Although the distribution isn't as even as just rolling one die, the distribution of rolling 1 through 6 is the same, only rolling a zero is different. For a major roll, roll tend to be something other than zero. For minor rolls, they tend to be zero.

Target Numbers:

Setting a target number for an action is up to the Game Master, though some guidelines are provided later on. To give some meaning to the numbers, descriptive adjectives are provided in order to give a sense of scale.

TN 09 -- Average/Easy
TN 10 -- Tricky
TN 11 -- Challenging
TN 12 -- Difficult
TN 13 -- Demanding
TN 14 -- Extreme
TN 15 -- Legendary

Those familiar with the Action! System should recognize the adjectives. The D6 MOD7 system uses some terminology from the Action! System game as provided by the OGL.

THE MATH:
Nothing extremely technical here, just the percentages of rolling certain things.

Major roll
Roll -- Chance to Roll -- Chance to = or exceed
0 ------- 0.138888889 ------- 1.000000000
1 ------- 0.143518519 ------- 0.861111111
2 ------- 0.143518519 ------- 0.717592593
3 ------- 0.143518519 ------- 0.574074074
4 ------- 0.143518519 ------- 0.430555556
5 ------- 0.143518519 ------- 0.287037037
6 ------- 0.143518519 ------- 0.143518519

Minor roll
Roll -- Chance to Roll -- Chance to = or exceed
0 ------- 0.166666667 ------- 1.000000000
1 ------- 0.138888889 ------- 0.833333333
2 ------- 0.138888889 ------- 0.694444444
3 ------- 0.138888889 ------- 0.555555556
4 ------- 0.138888889 ------- 0.416666667
5 ------- 0.138888889 ------- 0.277777778
6 ------- 0.138888889 ------- 0.138888889