It is extremely common for answers on this site to quote either "There are no hidden rules", "Spells do only what they say they do", or often both (example), almost always regarding 5th Edition Dungeons and Dragons. There are also a few questions asking where these principles come from (example and example) – the answers typically refer to Tweets by Jeremy Crawford (a designer of 5e).

But the quotes seem somewhat trivial to me. What could rules mean except what they say? And if rules are secret from both the GM and the players, they don't functionally exist for a tabletop game. So I wondered whether there were, or are, other famous RPGs that somehow broke these principles in a way I hadn't considered.

Things That I Don't Consider to be Answers

  • I know that Paranoia specifically tells its players not to read the rules. But I don't think this counts, mainly because the secrecy is mostly a joke (discussion here).

  • There are games like Mao where the rules are deliberately secret from some players, but again, not from all of them; also, figuring out the rules is the whole point of the game. Although I can easily imagine an RPG with a similar aim, I don't think it would really count as an answer.

  • All multiplayer RPGs have social rules, which are usually left implicit. "The aim of the game is for everyone to have fun", "Don't harrass or bully the other players or GM", etc. While important, these rules are never the target of "hidden rules" comments, and so aren't answers here.

  • Errata don't count, because the intention is that all players and GMs read them (and that future editions include them in the main rules).

  • Publishing new books with extra spells, classes, items, etc. does not really count, because those are additions to the rules rather than changes.

Do/did any major RPGs have "hidden rules" or "spells that do things other than what they say", in contrast to the common statements about D&D 5e?

If unsure what counts as a hidden rule, consider:

  1. Do I need to know this to effectively play my part of the game? A player doesn't normally need to know the exact statistics of monsters, but does need to know how the details of how their spells work.
  2. Is the information hidden from me (or at least is very difficult to find), in a way that seems unreasonable? If the information is in the GMs' Guide with a notice saying, "This is for GMs only", and the GM is supposed to not tell me if I ask, then it probably counts as hidden (unless, like Paranoia, players are supposed to read it, but lie and claim they haven't). But if the hidden information is clearly something that a player is supposed to work out by logic or experimentation ("What happens if I cast Fireball on this rock with a picture of a fire on it?", "How do we discover the murderer?"), that doesn't count because it is reasonable to hide it.
  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$
    – Oblivious Sage
    Commented Mar 27, 2022 at 19:32

9 Answers 9


“No hidden rules” is just what it sounds like, and what several other answers say: the rules are what’s printed on the page, and you have access to them.

That wasn’t always the case.

You mention Mao as “not an answer”. Oddly, that’s a good model for how many games used to work: the GM knows the rules, and the players don’t. or at least they only know some of them.

Up through about 2000, there was a weird taboo around players reading books that were intended for the GM. For example, from the preface to the AD&D DMG:

What follows herein is strictly for the eyes of you, the campaign referee. As the creator and ultimate authority in your respective game, this work is written as one Dungeon Master equal to another.

This sentiment shows up pretty often. So, everything in that book is intended to be secret from players. How that played out at any given table varied wildly, just like you’d expect, but the default version of D&D was a lot like Mao. One person had a rules reference, everyone else learned by doing.

Some things, that’s clear cut. For “how does alcohol work?” the answer is obvious: ask your DM. Because it’s in the DMG. But, more relevant to your question, sometimes it was more insidious. For easy example, take charm person.

I’m not going to copy the AD&D PHB text here, because there’s a lot of it. It’s basically what you’d expect: charm person for magic users refers back to the Druid’s charm person or animal, plus a list of things that are considered “persons”. Charm person or animal works about like modern charms: Subject regards you favorably, new saves if you threaten it, breaks if you attack it. It also had a periodic save with frequency based on intelligence.

Then, the AD&D DMG has this to say:

Charm Person: Attacks causing damage upon the subject person will cause a saving throw bonus of +1 per hit point of damage sustained in the round that the charm is cast.

If you’re playing “correctly”, you never see this rule. You might notice that charm never works in combat, but the mechanical basis is secret.

The DMG has a half-dozen similar entries for each spell level, for each class. So, if you’re coming from that background, “there are no secret rules” is extremely meaningful. It’s also revolutionary (or would be if that hadn’t been the new normal since the mid-noughts).

Most games before D&D’s third edition that I have read had similar attitudes. I’m not claiming that any change was WotC’s doing, but that was the timeframe and 3e reshaped the landscape in a big way.

So, bottom line: yes. There were secret rules.


Now, all that said, there was never any D&D police force that would haul you off to RPG jail if you read the DMG as a player. You can definitely find accounts online (horror stories usually) of DMs becoming very upset if they learned a mere player had dared read DM material, but I suspect most groups didn’t really care.

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    \$\begingroup\$ @Exempt-Medic You're right, I did say that. But I did find fectin's answer very helpful for understanding why people say that 5e has no hidden rules. Partly this is because the "do not read" message seems serious here (while in Paranoia, it seems players are supposed to read the rules, while pretending they didn't). And partly because I did not realise how widespread this practice was in old RPGs. I was trying to avoid a long list of relatively niche, experimental games; but this answer explains why the concept is embedded in RPG culture. \$\endgroup\$ Commented Mar 25, 2022 at 19:54
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    \$\begingroup\$ technically the secret handling of the spell resistance might also be considered a part of the monster statistic - which would not be known to players usually. In fact, in many table rounds I play with having the monster manual of the game on the table does get you evil eyes from the GM. \$\endgroup\$
    – Trish
    Commented Mar 26, 2022 at 9:39
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    \$\begingroup\$ This idea of "For GM's Eyes Only" was naive in one important sense: a GM of one game could easily be a player in another game. Are they supposed to ignore what they knew of these secret rules? \$\endgroup\$ Commented Mar 27, 2022 at 16:51
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    \$\begingroup\$ @ShawnV.Wilson there are a lot of downsides to that approach. Talking about the (many!) reasons I think it’s a bad idea seemed a poor fit for this answer (rpg.se embraces all playstyles; it distracts from the answer; no-one wants to read my novel-length essay on old school, etc, etc). So, yes, but I’m not adding that to the answer. \$\endgroup\$
    – fectin
    Commented Mar 27, 2022 at 21:11
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    \$\begingroup\$ Also from the AD&D DMG - "As this book is the exclusive precinct of the DM, you must view any non-DM player possessing it as something less than worthy of honorable death." Gary had very strong opinions about how the game should be played. \$\endgroup\$ Commented Apr 10, 2022 at 7:03

"Spells" that do more than they say are fairly common.

The easier answer is in "spells only do what they say". It's pretty common for game systems to give a character an unusual ability (whether the setting calls it magic, cybernetics, bending, superpowers, or whatever) that only describes a cause -- what action the character can take -- and then steps back and lets the players and GM figure out what effects that might have, with minimal guidance.

For example, in a game like Marvel Super Heroes (aka FASERIP), you might have the ability to generate electricity in your hands. The game may tell you how much damage that does as an attack, but you can use that ability to do virtually anything you can imagine a burst of electricity doing -- from starting a car to electrifying a puddle to drawing down a huge lightning bolt from an existing storm. The only real limit is what you and the GM agree to.

D&D, at least in 5th edition, doesn't allow things like that. Shocking grasp is very specific about what it does and how you can use it. You can't electrify a puddle. (4th Edition was also strict about what abilities could do, and I believe 3rd was as well; I can't really speak to anything prior to that.)

Similarly, a fireball creates no pressure wave, it can't burst doors or detonate like a block of C4 -- but many games do allow such shenanigans. It's not just a side effect, but the intended playstyle of the game.

"Hidden rules" are not a thing, but people act like they are.

I don't think truly hidden rules, in the sense of rules of the system that apply but are not stated in the rulebook, actually exist in any system. That's the point of a rulebook, after all, and how would such a hidden rule be discoverable for the person running the game? (Here I draw a distinction between a rule that's not stated anywhere and a rule that only the game-runner is meant to be aware of, which may well exist as a specific mechanic of a system.)

Still, some of the questions we get will act like there might actually be hidden rules. We often get questions that (in whole or in part) run along the lines of "The rules say thus-and-such but is that how this really works?" or "The rules don't say this, but --" and the statement about "no hidden rules" is meant to remind people that the rules are just what they say.

Things that are not "hidden rules" but kinda look like it

There are a few things that can look like "hidden rules" at first glance, but aren't.

Sometimes, we have situations where synthesizing several different rules from different parts of the book creates a rules scenario that's odd or ridiculous, but that's less a hidden rule than a surprising (and often apparently unintentional) interaction, which usually comes from hitting the edges of the abstractions we use to avoid simulating every aspect of combat.

As an example, I always think of the (in-)famous Peasant Railgun, which takes advantage of the Ready action as an abstraction of simultaneous activity, but then treats the result of that abstraction like a physics problem in order to calculate velocity and momentum. The Ready action isn't intended to break time and space, it just looks like it can if you extend it to an absurd degree.

Another source of apparently hidden rules can be unclear house-ruling. Some players don't have much rules knowledge and depend on the DM to tell them what they can and can't do. When their normal DM is fairly permissive of getting clever with spells and other special abilities (such as setting a grease spell aflame or letting water conduct lightning damage to shock a wider area) and they move to a new table, they may find that their new DM doesn't allow the tricks they've grown to depend on. It feels like there's a hidden rule -- but there isn't. It's just a house-rule that wasn't clearly stated to be a house-rule, and the player thought it was official.

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    \$\begingroup\$ It might be an idea to confine your statements about D&D to the editions to which it applies. Certain earlier editions did follow the philosophy that you could use a spell for more than its explicitly listed functions. \$\endgroup\$
    – GMJoe
    Commented Mar 25, 2022 at 7:52
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    \$\begingroup\$ I think this answer could be improved by providing a link to the Peasant Railgun concept/explanation. ;) \$\endgroup\$ Commented Mar 25, 2022 at 14:35
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    \$\begingroup\$ I think the biggest source of "hidden rules" is when people treat an RPG as a physics simulation. So "Grease is flammable" and "Lightning bolt electrocutes everyone in a lake" are rules that some GMs add because they like physics simulations, and then people bring those rulings to another table and treat them as Actual Rules -- which aren't in the rulebook. (And thus, hidden rules.) \$\endgroup\$ Commented Mar 25, 2022 at 18:16
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    \$\begingroup\$ "D&D, at least in 5th edition, doesn't allow things like that." I must point out that D&D, at least in 5th edition, doesn't disallow things like that. \$\endgroup\$
    – Mark Wells
    Commented Mar 25, 2022 at 20:15
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    \$\begingroup\$ "spells only do what they say" I must point out that D&D, at least in 5th edition, has no rule saying this. \$\endgroup\$
    – Mark Wells
    Commented Mar 25, 2022 at 20:39

Many games do not follow the "spells do only what they say they do" paradigm

At least in the sense that the phrase is usually used in D&D circles, the "Spells do only what they say they do" does not apply to many other RPGS.

To be clear, in the context of D&D 5e, the statement "Spells do only what they say they do", generally means that spells do not have unstated side effects. If a spell has a name or description that involves fire, but does not state that it will ignite flammables or something similar, then it will not ignite flammables. If a spell involving lightning does not describe an interesting side effect with water, then it will not have an interesting side effect with water. And perhaps best known, the grease from the spell grease is not flammable. (Notably, some earlier editions of D&D required more adjudication)

This is in contrast to more "freeform" magic systems. If a Mage in Mage: The Awakening invokes a forces based electricity effect, it may well have a cascade of side effects and interact with electronics and water in all kinds of ways. It is up to the Storyteller to determine the full implications and most players will expect the Storyteller to use analogies to how electricity in the real world would react.

While less freeform than mage, if a Tremere in Vampire: The Masquerade used The Lure of Flames the player would expect it to have realistic side effects even if the side effects were not described under the system section of the path of flame (P. 218 of the V20 edition). It implies that the flames from path of flame can start other fires, but it never directly states it. It also does not say that the flames will light a dark area, though I suspect most players will expect them to. The Storyteller is expected to adjudicate such side effects.

I am not aware of any games that use hidden rules as you mean them, but many games expect much more adjudication than DnD 5e assumes as its baseline

I am not aware of any games that use "hidden rules" in the sense I think you mean it. It would be infeasible, since as Mary points out in a comment, it would mean you could only have players that never GMed/DMed/acted as storyteller.

However, while Jeremy Crawford officially states there are no hidden rules in D&D, many other games deliberately require the storyteller to fill in a lot of gaps.

If you want to do a cross-over between Vampire: The Masquerade and Werewolf: The Apocalypse, it requires a lot of filling in the gaps by a storyteller even though they are theoretically compatible. Mage: The Awakening is meant to require storyteller adjudication for a lot of things of the type that D&D 5e would provide clear rules on, such as what a spell should do.

Also, many games (including D&D 5e, though arguably to a lesser extent than some others) have things in lore that the rules as written do not provide a path to copy. For instance, in the lore for Assamites (page 48, in the Vampire 20th book) includes the fact that Clan Tremere placed a powerful curse on the entire bloodline and uses that to explain the official weakness. I am not aware of any book for Vampire: The Masquerade that provides systems or rules for such powers.

So, while perhaps not "hidden rules", many games require the GM to do a lot of adjudication which will vary from one table to another and many games have facts in lore that are not well supported by rules as written.

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    \$\begingroup\$ It might be an idea to confine your statements about D&D to the editions to which it applies. Certain earlier editions did require more adjudication than modern editions. \$\endgroup\$
    – GMJoe
    Commented Mar 25, 2022 at 7:53
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    \$\begingroup\$ To follow up on GMJoe's comment. Pre-ADnD1e spell lists are good examples of dnd spells thst are much less precise in their wordings. Those lists are available from games like FMAG (for oDnd) and OSE' s online srd (for b/x). Heres a link to the magic user spell list if you're interested. oldschoolessentials.necroticgnome.com/srd/index.php/… \$\endgroup\$
    – 3C273
    Commented Mar 25, 2022 at 11:25
  • \$\begingroup\$ @GMJoe Fair point, I'll edit. \$\endgroup\$ Commented Mar 25, 2022 at 15:59

D&D 5e

I would say that, contrary to the saying, D&D5e has at least one hidden rule outlined in this question, What counts as a target for a spell? As mentioned in the question, spells, abilities, and rules all explicitly deal with spell targeting - Glyph of Warding requires spells stored in it must target a single creature or area, the War Caster feat requires a spell target only a single creature, the general rules about cover specify that you cannot target a creature behind total cover.

Given that all these abilities and more have specific requirements for targeting, you would think each spell would have explicit rules for what it targets - but they do not. Since the rules require you to know spell targets, but does not define them, I would say what spells target is effectively a 'hidden' rule.

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    \$\begingroup\$ I'm glad someone wrote this answer, and I'm sorry it's getting downvoted. 5e has tons of important rules that are well-known to experienced players but are never explicitly stated, arising instead from interactions, omissions, and context, often requiring a comprehensive and legalistic analysis of the source material to uncover. Consider dropping an item, unarmed vs. natural weapon smites, and reaction sneak attacks as additional examples. \$\endgroup\$
    – Joe
    Commented Mar 25, 2022 at 20:11
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    \$\begingroup\$ For what it's worth, I upvoted this. It agrees with the answer on Meta that I linked to, that claims that 5e seems to have hidden rules, even if it doesn't really. That didn't fully answer my question, nor does @IronWilliam's answer. But I think this is a good start to an answer like "Yes, 5e has several important rules that must be pieced together by forensic examination of the text, so they seem hidden to new players; here are some examples". \$\endgroup\$ Commented Mar 25, 2022 at 20:49

This answer echoes some parts of the one from @fectin, but I think I can observe some additional aspects of the earliest editions of D&D.

Many people have said that Original and Advanced D&D (1974-1979, hereafter O/AD&D), starting from hobbyist publications without editors, don't have the best organization or layout. A number of details to related topics are spread out among multiple books. As books were written and published, aspects of the game were changing rapidly and in an unorganized fashion (much like when rules get altered and errata'd in new 5E books, but with arguably greater variation).

In particular, there are several places in OD&D where some aspects of spells are given in the player's book ("Volume 1"), and other commentary in the DM's book ("Volume 3"), which players weren't supposed to read, even though these were delivered together in a single box. For example, from the DM's book (Vol-3, p. 9):

While some referees allow Fire Balls and Lightening Bolts to be hurled in confined spaces, blasting sections of the stone equal to the remainder of their normal shape, it is suggested that the confined space cause these missiles to rebound toward the sender, i. e. a Lightening bolt thrown down a corridor 40 feet long will rebound so as to reach its stated length of 6" (60 feet underground), and this will mean the sender is struck by his own missile...

That's not something (either the blasting-stone or rebound-on-caster variant) that players would know about from reading the spell descriptions in the players' book.

This is exacerbated in 1E AD&D. It's important to bear in mind that the 1E AD&D core rules weren't all published at once; they were released one at a time in 3 sequential years (due to difficulty publishing the first hardcover gaming rule books). So the MM came out in 1977, the PHB in 1978, and the DMG in 1979; and the author's sense of the game shifted in many ways in the intervening years. As @fectin points out, the DMG has a rather sizable "Spell Explanations" section (p. 41-47) given over to new spell adjudications that effectively serve as errata for many of the spells, and to which players are explicitly not allowed access. (One example among many: the DMG says the light spell can be used offensively on an enemy's eyes to blind them, when there's no hint of that usage in the PHB.) There are also sections on Aerial and Underwater combat which have extensive lists extrapolating the changes in effects in many spells in those environments, again, which the players don't have advance access to (example: lightning bolts underwater expand to the size of a fireball area, etc.).

This is all aligned with many other aspects of the game. Magic item effects are cordoned off in the DM's book (contrast with 4E which put them in the player's book). In 1E AD&D even the combat tables (chances to hit) were walled off from players and explicitly prohibited their awareness. Flasks of oil & holy water were on the player's equipment list for purchase, but the effects of their use were hidden in the DM's book (in OD&D, a player could easily have no idea that lamp oil could, unrealistically, be used as a weapon).

Furthermore, the original players/authors of the game had a tendency to argue that some heretofore unknown aspect of the game could be logically deduced from the existing rules structure. In particular, more than once I've seen them point to the fact that player-invented new spells are allowed, and then argue that for some critical use-case a previously unknown spell would necessarily have to exist in the game world.

Here's an example. In the old ENWorld Q&A on 2006-10-21, Gary Gygax (as "Col Pladoh") responds to a player's question about whether they can melt the whole Temple of Elemental Evil with a series of transmute rock to mud castings. Gary indicates that this wouldn't be possible, and that if a player in his game tried it, they'd be immediately cursed with some kind of wasting disease instead. He goes on (lightly edited for typos):

To stop the rules lawyers from their shrill protests I'd write up a few spells to cover constructions -- anti-disintegration, anti-rock to mud, etc. Also a few retributive spells to be activated and aimed unerringly at any spell caster attempting to bring down a structure by that sort of obvious and predictable tactic. Just because such spells are not included in the standard roster doesn't mean they don't exist.

Arguably these are "secret" rules for O/AD&D spells and as much as I love the early game, that's something I've personally been critical of in the past.

And here's another example, perhaps easier to concisely observe. From the 1E AD&D PHB, here's the start & end of the description to the top-level druid spell, creeping doom:

When the druid utters the spell of creeping doom, he or she calls forth a mass of from 500 to 1000 (d6 + 4) venomous, biting and stinging arachnids, insects and myriapods... There are a number of ways to thwart or destroy the creatures forming the swarm, all of which methods should be obvious.

What exactly are the "number of ways" to thwart or destroy the lethal magic insect swarm? They're not defined in the spell description or anywhere else; they are assumed by the writer to be "obvious". So there's definitely some part of the feature set of the spell which is explicitly not said by the spell description, and must be deduced externally by the players and DM. This is very much the sensibility of the early D&D game (prior to say, 3E).


Your Exclusion of paranoia is wrong because you mischaracterize what secret is in Paranoia.

Paranoia is all about secret rules...

Paranoia is a game that revolves around this. The GM is not only in the right to decide what the rules say or do (it literally says: the rules are wrong then!), the player is not even allowed to read the rule book. Reading the rulebook and admitting to it is treason. Players are not allowed to know the rules. Knowing the rules is useless anyway because the rules are wrong and the GM is always right.

If the GM decides on a whim in their infinite wisdom that starting now cover gets enemies a bonus of enrolling all bad dice to the one attacking whoever is in cover, then he is not only right, he does follow the rule to the letter, no matter what the rule actually says. And he doesn't have to announce it.

Also, being unhappy is treason. Are you happy not knowing the rules Citizen?! Of course, you are happy!

...where secret means not to admit to knowing.

While admitting knowledge of the rules is treason, Paranoia tries to foster a playtyle where players have secrets from the GM in some ways. There's no IC/OOC divide, the GM is clearly a hostile being to the players, so it fosters insubordination and trading of secret knowledge. This in turn means, that the label of secret is less for not knowing but more for not admitting to knowing.

This is even called out in some places: there are clear markers in printed campaigns where it would be more fun to stop reading as a player. Yes, there are lines like "but we know you'll read it anyway, you filthy traitors" and "no, seriously, you'll have more fun if you go in blind to this part". So either the GM is not assumed to read the stuff either, or the game is so incredibly tongue in cheek that they know players will be mutie commie traitors... Wait, that's exatly what they play!

Yes, Paranoia is mindfuckery, but it is not secret rules as in it's not spelled out here but means something other. It is all about You are not supposed to know the rules, and if you know, don't let yourself be caught and use your secret information to your advantage to play mindgames with the GM because I know that you know that I know that he too is a filthy mutie commie traitor.

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    \$\begingroup\$ -1. Paranoia is explicitly called out in the question as being not a valid answer to this question. Unless you intend this as a frame challenge? \$\endgroup\$
    – GMJoe
    Commented Mar 25, 2022 at 7:54
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    \$\begingroup\$ @GMJoe Challenge, because Paranoia calls it out that they expect players to be filthy mutant commie traitors that red the rulebook anyway, so secret isn't "not known" but "not supposed to know" for it \$\endgroup\$
    – Trish
    Commented Mar 25, 2022 at 8:11
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    \$\begingroup\$ "red" the rulebook... I see what you did there :P \$\endgroup\$ Commented Mar 25, 2022 at 18:06
  • \$\begingroup\$ @Trish I did link to another answer that also explains this concept, but thank you for going into more detail. \$\endgroup\$ Commented Mar 25, 2022 at 19:57
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    \$\begingroup\$ @Tektotherriggen "in the linked answer" is not "in the opener" and thus is simply not there. Questions need to stand on their own without reading links. \$\endgroup\$
    – Trish
    Commented Apr 5, 2022 at 15:09

I suppose there may be some argument as to whether it's a "major" RPG, but Planet Mercenary, by Hypernode Press, has special abilities that carbosilicate amorphs can gain in-game through certain actions with the following text in the book:

We recommend that the GC not tell the player they can do these things. Let the players respond to a need, and then grant them the associated skill.

  • \$\begingroup\$ That rule seems pretty clear and not hidden \$\endgroup\$
    – NotArch
    Commented Aug 15, 2023 at 17:42
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    \$\begingroup\$ Not hidden to the Game Chief (GC). I guess it's much like the AD&D examples where it's assumed that most players don't read how to run the game. :) \$\endgroup\$ Commented Aug 15, 2023 at 18:02

D&D 3rd Edition (but not 3.5) famously has the hidden rule that the Two-Weapon Fighting feat is only for Melee Weapons:

Two-Weapon Fighting [General]

Benefit: The character's penalties for fighting with two weapons are reduced by 2.

Where is this rule? Why, in the item description for the Heavy Crossbow, of course:

Heavy Crossbow

A heavy crossbow requires two hands to use effectively, regardless of the user's size. Loading a heavy crossbow is a full-round action that provokes attacks of opportunity.

A Medium-size or larger character can shoot, but not load, a heavy crossbow with one hand at a -4 penalty. A Medium-size or larger character can shoot a heavy crossbow with each hand at a -6 penalty, plus the usual -4 penalty for the off-hand attack (-6 primary hand/-10 off hand). The Two-Weapon Fighting feat does not reduce these penalties because it represents skill with melee weapons, not ranged weapons. The Ambidexterity feat lets someone avoid the -4 off-hand penalty (-6 primary hand/-6 off hand).


Even in D&D 5e, there are hidden rules

The statement made by Jeremey Crawford that there are no secret rules, was made in reference to reading any given rule to say something it does not, and he actually picked his word choice carefully in this case -- there may not be secret rules, but there certainly are many hidden ones.

Secret and hidden, while similar, are not quite the same thing. Secret has a connotation that implies other people are not allowed to know about it (Cambridge Dictonary), that is the author would be hiding it on purpose. Other than in D&D 1e's DMG which contained rules that were intended as secrets for the players, the authors are obviously not trying to do that any more, as evidenced by Jeremy's statement. Hidden however is weaker, it lacks the need for intent. It means something

that most people do not know about (Cambridge dictionary)

And in this sense, there are many hidden rules in fifth edition, rules that most people do not know about, or rules that are not obvious, but still a consequence of the written text. This site derives a good share of its raison d'etre from such rules.

Here is one example: there is no explicit chart in 5e that gives you height ranges in feet for different size categories. However, you can construct such a chart, by combining rules statements about the size range of medium sized characters and the rules for the enlarge/reduce spell, that changes size categories by doubling or halving height. So that information can be found in the rules, but it is not obvious -- it is hidden, and needs work to be uncovered.

Another example is where souls go after dying. This is not covered in the section about death, or the section about the planes in the rules, but is in there -- in the section about "Magic in Your World", not the first place most of us would think to look for it, so in a way, it is hidden.

A third example, there are rules that are implicitly assumed, without ever being explicitly spelled out, for example that mundane things behave as we would expect them to. This is somewhat obvious, otherwise you would have to describe all the implicit understanding of how the world works in the rules, which is impossible, but it never is said.

Just take a look at the active questions tab, and it is highly likely that you will find a Q&A where the answer can be derived from the rules text, but depends on triangulating from a number of sources in the rules in a way that requires a good amount of familiarity and expertise -- these are situations covered by the rules, but where the conclusion is not obvious, so in that regard, they are hidden.

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    \$\begingroup\$ I don’t think this is really what the question is asking for. It seems like the question is looking for hidden rules as a design element, not implications of written rules. Nothing you describe in this answer is actually a rule, rather, you’re listing implications of rules, which aren’t rules at all, just your interpretation of the rules. \$\endgroup\$ Commented Jun 25 at 9:48
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    \$\begingroup\$ Or in other words, if we’re having to synthesize rules together to get the thing you’re claiming is a “hidden rule”, that’s not a hidden rule, we’re just doing rules interpretation at that point. \$\endgroup\$ Commented Jun 25 at 9:52
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    \$\begingroup\$ @ThomasMarkov Likely true, this is not what the question had in mind. But it is an important point relating to the premise of the question, making it a frame challenge. Where to draw the line between a “rule” vs. an “interpretation of the rules” is notoriously contentious, but for my money the example about the height ranges of creatures of various size categories is something that is in the rules, and not a ruling or interpretation. \$\endgroup\$
    – KRyan
    Commented Jun 25 at 16:41

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