The clue probability rule

(I'll return to posting gameable content shortly.)

Most everyone on the internet is familiar with Alexandrian's three clue rule. The stipulation of that rule is that players are likely to miss some of your prepped content, which also includes any given clue. So to give the players a decent chance of progressing through a investigation-adventure, you need more than one clue leading to the conclusion, or to any breakthroughs that eventually lead to the conclusion. True to its name, the rule states that the perfect number of clues for each breakthrough is three.



I like this rule, for two reasons.

First, it means that each breakthrough is overdetermined: there is more than one path leading you there. This, to me, was a central thing in Masks of Nyarlathotep. You had a hunch from the beginning that you were going to Africa, but it felt very distant and hard to justify - what were you going to do there? With each new clue, you became more and more confident that you were actually going to Africa, up until the point where it seemed like the only reasonable thing to do. So basically, each new clue reinforces those you already have. Maybe you were already sure that Mr Vardis was a killer, but confronting him at this point seemed to carry so high costs. And then you learn that he is about to strike again, and suddenly acting becomes more urgent.

Second, and conversely, it also gives the adventure a branching structure. As there are always too many leads to follow up, the players will have to prioritize and consider what-if scenarios. Once the killer is caught they have perhaps seen 75% of your prep, so they still might not know what was the deal with Mr Vardis' prize-winning kennel. This makes the world seem bigger and the mystery more engaging.

In sum, the three-clue rule recasts a linear adventure to a dungeon-structure, where clues lead to new locations and breakthroughs in the investigation much like doors and corridors lead to new rooms.

However, this also shows some problems with the three-clue rule.

If we take the dungeon analogy seriously, we should accept that it isn't just a version of a linear adventure. Most dungeons don't have a straight-line progression to the end. And most dungeons are ok with you not reaching the end. If we were to apply these aspects of dungeon design, there would not always be three clues leading to the next revelation. Often there might just be one, and sometimes it would be hidden. And if the PCs don't find it they'll have to backtrack or even give up. And other times there would be clues that lead more than one step ahead in the investigation. So instead of 3 clues leading to A where 3 new clues lead to B etc, you'd have something more akin to a web.

However, even if we switch up our three clue investigation in this way - making sure that there are not just clues leading from A to B but also clues from B to A and from A to B via X and Y - we run into the problem of direction. Once you've reached C, it makes little sense to return to A because C is closer to the final conclusion than A. This is the same as in a dungeon, where you progress from the point of entry towards the central treasure through deeper and deeper levels.

If you want an even greater level of freedom in your investigation, your clues must be meaningful even in the presence of other clues. This is the rationale behind the guess-who structure and the triangulation (or venn-structure, as Kyana pointed out), described earlier. Here, all clues carry equal weight so as long as you're still uncertain about the solution, you gain from seeking and finding new clues.

For this type of investigation, I'd like to propose another rule for clues: the clue probability rule.

For each conclusion that you want the PCs to make, include a clue that they have a probability of finding whenever they make an effort to find it.

Say that you have a murder weapon: a knife with a easily recognizable look that would lead you to the smith who made it whom in turn could provide a list of clients. The knife must be somewhere. So we decide that it is in the pond, close to the scene of murder. Now if the PCs would look in the pond, they would definitely find it. So the clue probability is 100%. But perhaps they don't, because how would they guess that it is there? Instead, they might investigate the wounds. They can determine it's a knife without problem, but could they also conclude that this type of knife is made by Brambly the smith? I dunno. 1 in 6. Otherwise they just learn that the knife must have curious proportions. So maybe they ask a smith, or an assassin for help. 1 in 3. Or ask witnesses. 50% chance. The scrap-finder kids or pawn shops? 10% per day has passed since the murder. And so on.

The point here is that instead of designing several clues leading to the same conclusion (or in addition to), you design a clue that there's always a chance of finding whenever the characters make an effort to find it.

So now we have a hierarchy of blueprints for investigation adventures

The linear structure. A breadcrumb trail where A leads to B leads to C. Fail-safe if clues are not hidden behind roll to continue. Very little agency, built in story arc.
The three clue structure. A breadcrumb trail where multiple clues lead from A to B and multiple clues from B to C. Almost fail safe, given the plurality of clues to each conclusion. Some agency, allows for story arc.
The dungeon structure. A web of breadcrumb trails leading between A, B and C through multiple clues. Failure is unlikely. Good agency, story is largely emergent.
The open structure. A limited set of clues of equal (or similar) importance, which can be found through many different approaches. Failure is possible. High agency, no story arc.

Patterns, or Lessons from Isle of the Unknown

In many ways, Isle of the Unknown is a mediocre module at best. Its descriptions do little more than state that something is there, and content is repetitive and highly formulaic, as if procedurally generated from a limited set of items. And unlike Carcosa which benefits from a novel setting, Isle of the Unknown is essentially a French medieval island. Thus, it would be tempting to write it off as a dud existing only to meet a hexcrawl quota. But to me, the module is remarkable in how it presents you with the illusion of depth and meaning. I would argue that the formulaic elements conjure up a specific setting that the player-characters gradually learn to anticipate and understand. This is especially true for the magic-users that scatter the island. In Isle, it is fully possible (albeit improbable) to figure out both the location and overall theme of the not-yet encountered magicians once a few of them have been found.This alone makes Isle rather unique. But furthermore, the player-characters might even happen upon an encounter that explains the existence of some of the formulaic elements, thus enabling them to intuit some form of history for the setting itself. Now granted, the revelation is far from mind-boggling. But there is a "there" there, build into the adventure at a structural level and reflected in construction of the book. This makes Isle - flaws and all - one of the landmark modules of the OSR. It is designed to contain something more than what meets the eye.

A mystery.

One of the ideas that spoke most to me in the early OSR was that fantasy games had lost track of the fantastical. The more we play, the less is left unexplained. We have already fought a vampire, we know the HD of a goblin, and so on, and we are worse for it. Hence, the call to arms was to reintroduce mystery and a sense of wonder! And as soon as the problem had been outlined, so were its solutions, such as

- Don't speak in game terms.
- Use unique monsters, or at least pretend that they are unique by describing them instead of naming them.
- Go for weird instead of vanilla.
- Use pulp as inspiration, not Tolkien. And so on.

These are all good to very-good advice. But to me, they miss something fundamental. Neither of the standard solutions create MYSTERY. What they do is create uncertainty. Their idea is to replace some of the known with something unknown, or to recast the familiar as something unfamiliar. But at least to me, mystery is something completely different.

Mystery, I'd say, is the recognition of a logic that you cannot (yet) understand. It's a secret you can uncover, engaging a system that just might be there. A pattern. If this is true, then uniqueness and weirdness might, in themselves, actually be counterproductive. If the setting is systematically strange, it stops being unknown and becomes unknowable.


Following the cue of Isle, I'd argue that the solution to making a setting mysterious is not just to replace the ordinary with the extra-ordinary but to provide the extra-ordinary with its own pattern.