2d6 Based Core Mechanic Resolution System

This is going to be another one of those really geeky articles where I gush about numbers. The following is a disassembly of a basic D&D-style core mechanic based on a range of 2d6. This was developed as a foundation for my attempt at simplifying the mechanics of Traveller into something consistent and simple.

Gameplay revolves around a standard roll-over-DC core mechanic using 2d6 as the randomizer. Ability scores are determined by a 2d6 roll. Ability scores are also roll modifiers, there are no secondary statistics. On a check, the maximum result is 24. If you would roll greater than 24, you instead roll 24. The minimum result on a roll is 0. If you would roll less than 0, you instead roll 0.

The following chart shows all viable DCs for this game system. To use a DC outside of this range is a waste of time, as it is impossible to roll outside of this range. The DCs are categorized into named groups, called benchmarks, named by apparent objective difficulty. This chart can be used to quickly improvise DCs. Keep in mind that guaranteed DCs do not need to be rolled by anyone, as they are automatic successes for anyone with a score between 2 and 12, which is all PCs. Likewise, DC 25 is literally impossible, as the roll cap limits roll results to 24 regardless of combined modifiers. While apparently intuitive, this chart can be deceiving. Not all characters can hit every DC. A trivial DC for a character with a score of 2 is a guaranteed DC for a character with a score of 12. Rather, these benchmarks are spread out across the full possible range of PC scores. Thus, a PC with a score of 2 cannot roll higher than DC 14, and a PC with a score of 12 cannot roll lower than a DC of 14. 14 is the median DC, meaning it is the only DC within the range of all possible PCs. DCs below 14 include more PCs as guaranteed passes, and DCs above 14 exclude more PCs as guaranteed fails.

DC Ranges & Benchmarks
1	Guaranteed
2
3
4
5	Trivial
6
7
8	Easy
9
10
11	Moderate
12
13
14	Hard
15
16
17	Very Hard
18
19
20	Improbable
21
22
23	Nigh-Impossible
24
25	Impossible

This table presents a much more thorough account of the probabilities underlying any given possible challenge the DM may present. The highlighted score column is the most likely score. The highlighted DC is the range overlap for all possible scores. This table is much more useful for tailoring challenges to a specific character.

In addition, the probability that a character may have a score is given in the individual score column headers. This can be used to plan challenges in the absence of any specific character. When you select a DC, add together the score header probabilities for all cells marked in red or blue for that row. Depending if the rows are blue or red, this will give you the proportion of PCs who will auto-pass or auto-fail the chosen DC, respectively. For example, for a DC of 11, scores of 12, 11, 10, and 9 will auto-pass. The proportion of characters with a score in that range is (3+6+8+11) 28%. That means 28% of characters would auto-pass the proposed challenge. If that seems to high or too low a proportion of the human population, simply change the DC.

Check Probability Percentage by DC vs Score
		Scores
		1 (0%)	2 (3%)	3 (6%)	4 (8%)	5 (11%)	6 (14%)	7 (17%)	8 (14%)	9 (11%)	10 (8%)	11 (6%)	12 (3%)
Difficulty Classes	1	100	100	100	100	100	100	100	100	100	100	100	100
	2	100	100	100	100	100	100	100	100	100	100	100	100
	3	100	100	100	100	100	100	100	100	100	100	100	100
	4	95	100	100	100	100	100	100	100	100	100	100	100
	5	92	95	100	100	100	100	100	100	100	100	100	100
	6	83	92	95	100	100	100	100	100	100	100	100	100
	7	72	83	92	95	100	100	100	100	100	100	100	100
	8	58	72	83	92	95	100	100	100	100	100	100	100
	9	42	58	72	83	92	95	100	100	100	100	100	100
	10	28	42	58	72	83	92	95	100	100	100	100	100
	11	17	28	42	58	72	83	92	95	100	100	100	100
	12	8	17	28	42	58	72	83	92	95	100	100	100
	13	3	8	17	28	42	58	72	83	92	95	100	100
	14	0	3	8	17	28	42	58	72	83	92	95	100
	15	0	0	3	8	17	28	42	58	72	83	92	95
	16	0	0	0	3	8	17	28	42	58	72	83	92
	17	0	0	0	0	3	8	17	28	42	58	72	83
	18	0	0	0	0	0	3	8	17	28	42	58	72
	19	0	0	0	0	0	0	3	8	17	28	42	58
	20	0	0	0	0	0	0	0	3	8	17	28	42
	21	0	0	0	0	0	0	0	0	3	8	17	28
	22	0	0	0	0	0	0	0	0	0	3	8	17
	23	0	0	0	0	0	0	0	0	0	0	3	8
	24	0	0	0	0	0	0	0	0	0	0	0	3
	25	0	0	0	0	0	0	0	0	0	0	0	0

Using both the benchmarks and probability table together will yield the best results. Considering DCs in terms of what portions of people are likely to experience no difficulty, automatically fail, or have varying results, is much more useful than considering how much “effort” a single character might be circumstantially exerting at a single attempt on a task.