The d20, of course, is closely associated with the D&D game. But it's easy to forget that the early classic editions used the d20 in fairly narrow circumstances: just attacks and saves, in fact. Everything else about running an adventure was done with d6's -- it wasn't until the 3E version of the game that all of these other functions were replaced with the "core mechanic" of d20's for everything.
There's something about that I really like, in fact. (And I've written about it several times.) The granularity of the game is usually at the d6 level, except when death is on the line (an attack or saving throw), at which point the detail "zooms in" to the d20 level. That's a bit like a movie slowing down when a character is in mortal peril, simulating the cognitive effect from increased adrenaline. When everything else is d6's, they probably don't show up to clutter the character sheets. There's a small enough list of modifiers that the DM can just remember them all. And when things go off-book -- like they should -- it's easier to correctly estimate the real-world chance of success out of 6 pips than out of 20 points.
Here's an encyclopedic run-down of the use of the d6 for adventuring function in Original D&D. The majority of these cases appear in little brown book Vol-3, the DM's guide analog:
- Damage from falling into pit: Occurs on 1-2 on d6 (suggested example). (p. 5)
- Random dungeon stocking: Monsters appear 1-2 on d6. (p. 7)
- Random dungeon stocking: Treasures appear 1-3 on d6 with monsters, or 1 on d6 without. (p. 7)
- Searching for secret passages: Success 1-2 on d6 generally, or 1-4 on d6 for elves. Or elves can possibly find one 1-2 on d6 just by walking by. (p. 9)
- Opening doors: Success 1-2 on d6, or 1 on d6 for smaller characters. (p. 9)
- Spiking doors open: Success 5-6 on d6. (p. 9)
- Traps activating on trigger: Occur 1-2 on d6. (p. 9)
- Listening for sounds: Success 1 on d6 for humans, or 1-2 for elves, dwarves, and hobbits. (p. 9)
- Surprise: Occurs 1-2 on d6 unless some signal prevents it. (p. 9)
- Wandering monsters in dungeon: Occur on a 6 on d6, rolled each turn. (p. 10)
- Monsters continuing pursuit: Occurs 1-2 on d6 when party passes a corner, door or stairs; or 1 on d6 when party passes through a secret door. (p. 12)
- Castle occupants turning out: Occurs 1-3 on d6 within the castle hex, 1-2 at 1 hex distance, 1 at 2 hex distance. (p. 15)
- Becoming lost in wilderness: Occurs on either 1, 1-2, or 1-3 on d6, depending on terrain type. (p. 18)
- Wandering monsters in wilderness: Occur on 4-5, 5-6, or 6 on d6, depending on terrain type. (p. 18)
- Castle inhabitants pursuing party: Occurs 1-3 on d6 if hostile, or 1 on d6 if neutral towards party. (p. 19)
- Damage from fall off ship rigging: Occurs at one low pip on d6 for every ten feet fallen (more detail below). (p. 31)
- Ship crew in melee obeying other commands: Occurs 1-4 on d6. (p. 32)
So let's take stock of what we have there for a "core d6 mechanic" sensibility. We've found 17 cases in OD&D Vol-3. We note that in 14 of the cases success is indicated by a low roll (82%), whereas in only 3 cases is success indicated by a high roll (18%).
Clearly, the fundamental instinct of the writer with these d6 mechanics is for the low roll to indicate success. That doesn't mean low is "good" exactly -- consider a trap being sprung or a party being surprised, for example. But generally some new-thing-of-note pops up with a low d6 roll -- a change to the status quo.
Of course, since most of these mechanics have a base success of 1-2 on d6, the inverse is in the majority, and we might say that's thereby the status quo by definition.
The falling-off-ships-rigging case is interesting (Vol-3, p. 31), because it highlights that either side of the random occurrence could have certainly been phrased as the event of interest. In this case, the rules text is phrased in terms of taking damage from a low roll ("one chance out of six for every level fallen that damage will be sustained"), whereas the associated example is phrased in terms of saving with a high roll ("i.e. a fall from 40 feet will require a 5 or 6 to save").
And in the list above I'm not even counting cases from other books, like the end of OD&D Vol-2 (the monsters & treasure book), in which a roll of "1" on d6 bumps an individual gem in a batch up to the next-higher price level. Whereas, if you were in a different headspace, you'd likely think that a high value would indicate, well, a high value.
So where do the 3 outliers come from? To be clear, those are: (1) spiked doors failing, (2) wandering monsters in the dungeon, and (3) wandering monsters in the wilderness. Let's ignore the first of those for now. But the two wandering monster cases have a clear source -- that's exactly the mechanic in the earlier Outdoor Survival board game for the chance of a daily encounter (where the rule is itself optional). This is in contrast to all the other mechanics in Outdoor Survival, which are notably roll-low-on-d6 to break status quo -- e.g.: getting lost, finding food, or finding water (as allowed in some scenarios; finding food or water occurs 1-2 on d6 when permitted). Note that the lost/food/water mechanics are printed on the Scenario cards there, whereas the optional Encounters rule is in the separate rules pamphlet -- so they weren't synchronized with any core mechanic. And that's exactly why these rules appear in the same form in OD&D, since they were just wholesale lifted from that source & tweaked a bit. Examples below:
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| Outdoor Survival, Scenario 3: Search rules |
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| Outdoor Survival, Rules of Play: Optional Encounters rule |
So if I were going to get my rules-design steam press, and iron out the wrinkles in this particular system -- for both Original D&D and Outdoor Survival, because the system is at least conjoined if not identical at their root -- what I'd do is swap around the wandering-monster rolls and make them appear on low results on a d6, e.g., a "1" on d6 for encounters in the dungeon. Same for spikes failing, too. Taking the opposite tack and saying you're going to flip all the d6 rolls around so a high result is success entails a lot more editorial fixup-work (e.g., as Menzter tried to do with opening doors in BECMI; and as I've also stumbled towards doing in the past myself).
In conclusion, there's also a number of things that are attractive about what I might call an "accuracy" roll-low core mechanic. Principally, it's that announcing a target number is simultaneously communicating the probability of success. (As opposed to a high-roll mechanic, where the conversion between the two requires subtraction and then an off-by-one adjustment.) I assume that's why the writer of OD&D fell into this habit; you don't even need to mentally distinguish which way you're thinking about it as you furiously pound out the rules text on your typewriter. Additionally, to my mind, the die-roll then has the feel of communicating the amount of "error" in your task attempt, which is a statistically robust concept; as opposed to (I guess) "goodness", implied by a roll-high mechanic.
So there are days when I wistfully daydream of a D&D tradition in which all the mechanics were always roll-low by default, instead of the legacy we have. Imagine celebrating being "Number one!" on an attack roll with as much gusto as we now do a "Natural twenty!". (Although I suppose it might not be immediately as clear that an exotic die type was in play.) It would also synch up with the old roll-under-ability mechanic, which at one point seemed natural and obvious (rather than convert to a modifier, and now have many people ask, "why do we record ability scores anyway?").
Do you agree with the suggested roll-low tweak to wandering monsters in OD&D? Did I miss any notable d6 mechanics in the DM's rules for OD&D?
