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.

1d6 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%

2d6 Rolls

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%

Using these tables

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%.