To everything turn, turn, turn

The universal mechanic that was hiding in D&D all along!

There are a lot of ways that DMs have turned to over the years since the D&D white box in order to adjudicate various actions players want to take that aren’t covered explicitly in the rules. While there are definitely defenders who claim part of the charm of old editions is that every way of adjudicating something in the game required its own idiosyncratic sub-system, over the years a lot of DMs have spent a lot of time and energy trying to come up with a universal mechanic, if not to replace any of the “core” mechanics at least to fall back on when there isn’t a clearly defined procedure in the rules.

One of the commonest stabs at this universal mechanic is “ability checks”, usually against the characters’ attributes. Vague Countries has a nice discussion here.

The classic method, enshrined in Tom Moldvay’s Basic D&D (p. B60) is just to roll d20 below an attribute the DM picks. On the one hand, it’s nice and simple, on the other it really makes attributes much more important that they are in OD&D or in other parts of the rules; instead of a 16 granting a mere +10% on a d20 roll it suddenly becomes an 80% chance of success. Another method, apparently used a lot by Gary Gygax and Rob Kuntz is roll 3, 4, or 5d6 under an attribute, depending on how hard the task is. Dan “Delta” Collins has an analysis of the odds of the various rolls here.

But aside from the various complaints about the odds and the inflation of importance of attributes, generally speaking I find attribute checks not particularly satisfactory. It strikes me as a problem that most of them neither scale well against harder and easier tasks nor take into account level, which is the overall scale of competence that D&D is built on.

Recently, though, I’ve realized that there has been an almost perfect universal mechanic hidden in plain sight in D&D ever since the white box: I’m talking about the Clerical Turning mechanic!

Here’s how it was presented in the white box, rolling 2d6 on the following table:

Typical of Gary’s approach to rules, it presents as a table something that’s actually a simple formula, but that’s by no means obvious shorn of the numbers. The columns are actually the cleric’s level, 1-8, and the rows are the monster’s hit dice, 1/2 through 7. So really what this is presenting is that clerics have a Target Number of 7 against undead 1 hit die less than them, and it gets 2 points harder for each additional hit die the undead has, and 2 points easier for each hit die less. If the number is below 7 turning is automatic, and if it’s literally impossible to fail the undead is destroyed; similarly if it’s impossible for the cleric to succeed, the result is No Effect. Building in the automatic success, critical success, and automatic failure in this way is really sweet, and pegging the target below which you don’t even need to roll to better than 50% chance of succeeding really speeds up play, in my experience.

	HD	1	2	3	4	5	6	7	8
Skeleton	1/2	7	5	3	1	-1	-3	-5	-7
Zombie	1	9	7	5	3	1	-1	-3	-5
Ghoul	2	11	9	7	5	3	1	-1	-3
Wight	3	13	11	9	7	5	3	1	-1
Wraith	4	15	13	11	9	7	5	3	1
Mummy	5	17	15	13	11	9	7	5	3
Spectre	6	19	17	15	13	11	9	7	5
Vampire	7	21	19	17	15	13	11	9	7

So here’s the thing: here we have a method of comparing a character’s level with a target difficulty. For undead it’s just their Hit Dice, but you could imagine it being the dungeon level a hazard or lock is found on, or any sort of ad-hoc decision by the DM. What’s more, the 2d6 scale fits in nicely with attribute bonuses ranging from -3 to +3 as per Moldvay and its descendants. +/-1 is not quite as good/bad as being 1 level higher, +/-2 is equivalent to a level, and +/-3 is a bit better than being a level higher/lower. That seems pretty nice to me.

But wait, there’s more! How much does each bonus improve your chances of hitting the Target Number? Here’s a quick chart:

Total	Exact	At least	+1	improvement	+2	improvement	+3	improvement
2	3%	100%	100%	0%	100%	0%	100%	0%
3	6%	97%	100%	3%	100%	3%	100%	3%
4	8%	92%	97%	6%	100%	8%	100%	8%
5	11%	83%	92%	8%	97%	14%	100%	17%
6	14%	72%	83%	11%	92%	19%	97%	25%
7	17%	58%	72%	14%	83%	25%	92%	33%
8	14%	42%	58%	17%	72%	31%	83%	42%
9	11%	28%	42%	14%	58%	31%	72%	44%
10	8%	17%	28%	11%	42%	25%	58%	42%
11	6%	8%	17%	8%	28%	19%	42%	33%
12	3%	3%	8%	6%	17%	14%	28%	25%

The improvement in probability of success isn’t uniform, but you can see that the biggest differences fall right at the fat part of the distribution. It’s a bigger difference on your average roll than on the extremes, not surprisingly, and none of them are over 50%, so not overwhelming. Even nicer is that at best, a +1 is adding about 1/6 to your chances, a +2 is adding about 2/6, and a +3 is adding not quite 3/6. It could hardly be easier to remember or reason about.

To me this is actually pretty amazing: Roll 2d6 vs Target 9 against things that are even-on with the character in terms of level/hit dice, adding in any attribute modifiers, and Bob’s your uncle! If I were coming up with a mechanic de novo, I might be inclined to make even-on a target 7 but I can see an argument that if you have no particular reason to be good at a task it’s realistic that it’s more likely than not you’ll fail. I’m tempted to use Target 7 anyway as just being a little easier to remember, and being a bit more like the way combat works, with Level 1/HD 1 attackers being about 50-50 to hit unarmored foes, but I’m not sure whether I like Clerics vs. Undead then being a special case…

So there you have it, my new go-to Universal Mechanic for all older editions of D&D and their kin.