Classic Traveller resolves many actions by random numbers generated by 6-sided dice, typically 1d6 or 2d6. Of these, the 2d6 die roll is the most common. The problem with this system is that it does not generate a value spread where each value probability is equal. For example, the d20 has an equal probability of generating any integer between 1 and 20. This is useful because the average GM can easily work with percentage success rates in task, and a d20 can easily be converted into a percentage by multiplying by a factor of 5. Thus, if the GM determines that a player has a 25% change of accomplishing a task, it is a small matter to covert this to a d20 roll. A roll of 1-5 on a d20 represents a 25% probability.

With the 2d6 system, converting to a percentage is not so easy. Various values are more or less likely to occur, depending the the value in question. Here is a chart which relates percent probabilities to 1d6 and 2d6 rolls.

Die Value | Chance of rolling exact number |
Chance of rolling a number and above or below |
||
---|---|---|---|---|

1 | 16.7% | 1+ | 6- | 100% |

2 | 16.7% | 2+ | 5- | 83.3% |

3 | 16.7% | 3+ | 4- | 66.7% |

4 | 16.7% | 4+ | 3- | 50.0% |

5 | 16.7% | 4+ | 2- | 33.3% |

6 | 16.7% | 6 | 1 | 16.7% |

Dice Value | Combinations | Chance of rolling exact number |
Chance of rolling a number and above or below |
||
---|---|---|---|---|---|

2 | 1 | 2.8% | 2+ | 12- | 100% |

3 | 2 | 5.6% | 3+ | 11- | 97.2% |

4 | 3 | 8.3% | 4+ | 10- | 91.7% |

5 | 4 | 11.1% | 5+ | 9- | 83.3% |

6 | 5 | 13.9% | 6+ | 8- | 72.2% |

7 | 6 | 16.7% | 7+ | 7- | 58.3% |

8 | 5 | 13.9% | 8+ | 6- | 41.7% |

9 | 4 | 11.1% | 9+ | 5- | 27.8% |

10 | 3 | 8.3% | 10+ | 4- | 16.7% |

11 | 2 | 5.6% | 11+ | 3- | 8.3% |

12 | 1 | 2.8% | 12 | 2 | 2.8% |

These tables can be used for any number of tasks in Traveller, from building caracter generation tables to resolving player actions. If the GM, for example, determines that a player has a 30% chance to succeed in a task, he might elect to have the player roll 2d6. A roll of 9 or better, or 5 or less would indicate a percentage value of 27.8% or about 30%.