Dee-Five-Six (d5/6) and the odds it gives

This post gives a name to a very fast method of using dice that is already found in several games. It points out the similarity of the odds of this method to the even more widespread 2d6+/-modifiers method as well as current iterations of the d20 method.

The familiar 2d6 method with a target of 10

Many role-playing games resolve uncertainty with 2d6 rolls and a target number of 10 for success. You add the results of two six-sided dice and add modifiers, positive or negative, and aim for a total of 10 or higher to succeed (or to succeed fully). Typically, the modifiers are equal to, or derived from, one or more character stats. There may be more modifiers, plus or minus, for situations advantageous or disadvantageous. I will call this the 2d6/target10 method.

In this system, the default, when you have no modifiers, is a 1-in-6 chance of succeeding, because there’s a 1-in-6 chance of rolling 2d6 to get a total of 10 or higher. With bonuses, the odds of success increase as follows:

+0 (no bonus)	+1	+2	+3	+4	+5
17% (1 in 6)	28%	42%	58%	72%	83%

It is easy to do addition with small numbers, but there is still a step where players have to think about numbers and maybe ask questions about which modifiers apply, a moment in which the game action disappears behind rules talk.

The d5/6 dice pool method

 Another, modestly popular way to work out success and failure is with dice pools of d6 where any 5 or 6 is a success. You don’t count dice or “successes.” To succeed you just need any 5 or 6. With this method, the GM does not make up difficulty numbers on the fly, but trusts the dice for the fiction.

What’s great about this system is that players instantly see if they succeeded or not simply from the icons on the dice. Any single 5 or 6 means they succeeded. Even though the addition of several small numbers is easy, the instant recognition of an icon is easier still. Also, the target is always fixed, so players do not need to check with the GM each time they roll. This is very fast at the table.

I hereby dub this the d5/6 dice pool system (“dee-five-six”).

Notice in the table below that adding one die in the d5/6 dice pool gives odds tolerably like each added +1 in the 2d6+modifier system above.

0 (on 6 only)	1d5/6	2d5/6	3d5/6	4d5/6	5d5/6
17%	33%	55%	70%	80%	86%

 As you can see, I added a “less than one” or “zero dice” level. By “zero dice,” I mean zero d5/6s: players still roll 1d6, but success occurs only on 6 (not on 5). This level is exactly equivalent to the target of 10+ on 2d6 added together. 

Thus, the d5/6 pool method approximates the 2d6+mods method, although it gives slightly better odds. You could say it is slightly less punishing to PCs but it applies to NPCs, too. The difference is trivial.

In some 2d6/target10 games, like Maze Rats by Ben Milton, circumstances or character traits may let you roll “with advantage”: you roll 3d6, drop the worst die result, and add the remaining two and the bonuses. The odds of 2d6/target10 with advantage:

0	+1	+2	+3	+4	+5
36%	52%	68%	81%	89%	94%

This closely approximates the difference in odds when you add one die in the d5/6 dice pool method. Advantage with 2d6 thus has an effect similar to adding one die in the d5/6 dice pool.

Comparison with d20 systems

If you look at the widespread D&D-style d20 method, in which players roll 1d20+/-modifiers and the GM makes up target numbers or “DCs” on the fly, the distribution is similar. Here I present the DCs from Kelsey Dionne’s ShadowDark game. Each of the default DCs corresponds to both to the pluses in the 2d6/target10 method and to the d5/6 dice totals, as follows:

2d6target10 bonus	0 bonus	+1	+2	+3	+4
the odds	17%	28%	42%	58%	72%
+advantage	one column >	36%	52%	68%	81%
ShadowDark	DC18	DC 15	DC 12	9
designation	Extreme	Hard	Normal	Easy
unmodified odds	15%	30%	45%	60%
d5/6 pool	17% (zero d5/6)	33%	55%	70%	80%

In ShadowDark and other d20 games, you also have pluses and minuses (usually pluses), which this table does not take into account. If a typical character rolls with a +2, however, it works out as equivalent to the d5/6 pool method for whatever counts as “Normal” or “Easy” difficulty in ShadowDark. Extra bonuses make it easier, which is like adding dice to a d5/6 dice pool—and that makes sense.

If variations in odds of 5% or 10% make a big difference to you, the small discrepancies will matter. To me, they do not.

Here’s how the d5/6 pool works in a game system:

Rate abilities (stats, skills, talents, etc.) in d5/6 dice. Every point or die in a stat rating contributes one die to d5/6 dice pool, in which any 5 or 6 signifies success. Default levels will depend on the tone and genre of the game.

If you have no ability at a task normally requiring an ability or skill, but there is a chance you could pull it off, you have zero d5/6s: you roll 1D6 and succeed only on a 6.

“Advantage” adds one die to the d5/6 pool, too. Take a die away a d5/6 for “Disadvantage.”

The GM never needs to call out the target number. Players do not need to ask. Less thinking about odds for everybody, more attention to the action.

If you like, all 1s means a critical failure. Two or more 6s means a critical success.

What can you do with this? Lots of things. Here is one example. Let’s say you are playing Maze Rats, which uses the 2d6/target10 method to resolve risky situations. In that game, stats are rated 0 to 4. That includes the bonus to hit, which works in effect like another stat. The odds are minimally changed if you turn the stats into a d5/6 dice pool and roll that way instead. Advantage or disadvantage means adding or dropping a die from the pool.

As with all game systems, simulating combat, injury, and especially armor, is tricky. There are many functioning ways of dealing with all of that in a d5/6 dice pool method, but I will not get into that here. You can look at some of the game systems below, though, to see some examples of how it has been done. 

Game lineage. Game designers should be giving credit where credit is due and also keeping the history of our games alive.

I invented the name d5/6 dice pool to talk about it more easily, but I did not invent the method. It already has a following and I want to sketch its little-known history.

In the year 2000, a very light ruleset appeared called the One Braincell RPG, which, as far as I know, was the first to use a d5/6 dice pool system. I have never seen a complete copy of that game myself, and the author was apparently anonymous, but I know about it because Norbert Matausch acknowledged it as the source of his method in his very light minimald6 system, which spawned many genre versions by various authors (collected here by Yochai Gal). Independent game designers publishing open-access games apparently can’t be bothered to put dates on their games, so the chronology of these games is mostly lost, and I cannot give a timeline, but it looks like the minimald6 games are mostly from the mid-2010s.

Brandon McFadden’s original Tiny Dungeon rules of 2014 did the same thing as the minimald6, without any acknowledgement of One Braincell or another game. Alan Bahr’s Tiny Dungeon 2e (2018), “based on the game Tiny Dungeon by Brandon McFadden,” is a breezier version of McFadden’s original. Bahr has published many Tiny D6 variants since then for different genres, basically the same rules reprinted over and over with different dressing and some notably interesting variants. Scotty McFarland’s EZD6 game (2022) was clearly heavily inspired by this rules lineage, too, but it works with target numbers set by the GM and fewer dice.

If you are interested in the d5/6 dice pool method, check out the games just mentioned.

Going back further, the One Braincell rules clearly owe much to the Over the Edge game (Jonathan Tweet and Robin Laws, 1992), where traits are modular and where the system of an extra die for advantage is found, and to the game Risus (S. John Ross, 1993). The dice pool method goes back to Shadowrun (Jordan Weisman et alii, 1989), where the GM calls the target number for the dice pool roll, which was inspired by the Ghostbusters role-playing game (Sandy Peterson, Lynn Willis, and Greg Stafford 1986). Here we find deeper roots in Tunnels & Trolls (Ken St. Andre, 1975), one of the few true fonts from which non-D&D systems flowed, including Fighting Fantasy and thence Troika!. In the search for “minimal” tabletop role-playing game rules, it is hard to find anything earlier than Tunnels & Trolls, which was deliberately supposed to clean up and simplify the original D&D.

In hindsight, the obscure One Braincell RPG seems to have had unexpected influence. If anybody has a copy, send it to me, please.

The rules for Advantage and Disadvantage in D&D 5e are basically dice pool variants. I dug into the history of that here.

David McDaniel, a.k.a. Ted Johnstone, a.k.a. Tedron, first devised the convention of referring to dice as d(number), such as d6 or d10, "dee-six" or "dee-ten."