A d6 Skill System: 7-in-6 odds

This is a skill system based on old school x-in-6 checks which allows for effectively endless scaling. Obviously, 7-in-6 odds doesn't make much sense, but you're here reading this now so I owe you an explanation. It'll make sense soon.

This is intended for the sort of skills a Thief uses. Climb Sheer Surfaces sort of things - abilities that can't necessarily be attempted by everyone, or if they can, the skilled character gets an extra chance to do it beyond the normal level. I listed several more examples under the Specialist.

The original Thief skills table, a la Greyhawk (1975).

The basic idea here is to have a skill rating be a number from 1 to 6 that represents your odds of success. There are two problems* with this. The first is that 6-in-6 makes failure impossible, and the second is that 83.3% is hugely different from 100% so to stop it there feels like there is a lot more room for progression.

*Whether or not these are problems is subjective.

The twist is to not cap progression, but to soft-cap it instead. Rolling a 6 is always a failure, so with a skill of 6 you still only have 5-in-6 odds, but your leftover 1 rolls over to another die, so you need to fail 5-in-6 then 1-in-6 to truly fail. Thus, at a skill rating of 7, 2 points roll over. At skill 8, 3 points roll over, and so on. By skill rating 11, you hit 6 on your second die and carry the 1 again.

So that's how we do "7-in-6". 5-in-6, carry 2.

Skill Level	d6 Odds	Percentile Equiv.
1	1 in 6	17%
2	2 in 6	33%
3	3 in 6	50%
4	4 in 6	67%
5	5 in 6	83%
6	5 in 6 + 1 in 6	86%
7	5 in 6 + 2 in 6	89%
8	5 in 6 + 3 in 6	92%
9	5 in 6 + 4 in 6	94%
10	5 in 6 + 5 in 6	97%
11	5 in 6 + 5 in 6 + 1 in 6	98%

This works out neatly because, in essence, every 5th skill rating you get one extra attempt and the probabilities become more granular. Though it's absolutely worth noting here that a 5-in-6 is already high-level (equivalent in probability to a Master Thief!), so getting multiple roll-overs is ultra-high-level. Over 10, we're talking about less-than-halves of a percent. It scales infinitely but never hits 100%, though a skill rating of 25 means you need to roll a Yahtzee with all sixes in order to fail (that's 1 in 7,776).

Another really cool thing about this is that it maintains the existence of suspenseful moments at higher levels. A 5-in-6 doesn't feel suspenseful because you know you'll probably win. If anything, it falls flat when you fail an expected success. On the flip side, in this system, if you fail your first roll you now feel suspense on the second roll because this time it feels like you really can fail. In fact, for the first few levels above 5, the odds are really high that you're going to fail the second roll, but you might just scrape through - that brings back some of the suspense from early levels (albeit less frequently, which makes it more memorable when it happens).

Notably, skill rating 14 is the point where your failure rate is approximately 1% (actually 1-in-108). Each 5th skill point is a notable breakpoint where your failure rate halves between levels.

The elephant in the room here is that I haven't defined the actual progression rate for the skills. Oddly, I don't think this actually matters as much, but using the table above you can very easily convert odds up to 98% to their nearest equivalent. For my own games, I'm yet to nail down the exact progression I want.