How much can you change OD&D...?

[Finarvyn]

Sort of a rip-off from my earlier post in the “statless OD&D” thread, revised to make a topic of discussion of its own…

Here’s a question of philosophy: "How much can you change OD&D and still call it OD&D?" I don't really expect to get a definitive answer, because I don't think there is one. I thought of this question earlier today when I was tinkering with brown book classes and tweaking things to get a balance I liked better. Then I wondered if it was still OD&D.

vladtolenkov said:

Finarvyn's concern that we'd end up with something that wasn't quite D&D anymore is well justified.

And you know this is EXACTLY the sort of the thing that led to the creation of the rpg industry as we know it. Because what we call "house ruling" is often actually game design. Gygax's assumptions that the players would "make it up" just like he did-- led to lots of people doing just that and sometimes they wandered into places that stopped being what we might recognize as D&D.

And sometimes they were people like Greg Stafford or Steve Jackson.

These observations might better belong in the philosophy section but anyway...

* Clearly, one can use Chainmail combat or “Alternate” combat and it’s still OD&D.
* One can use all d6 for hit dice, or allow variation in dice type and it’s still OD&D.
* One can stick to the traditional 3-4 classes or add numerous extra classes (Paladin, Assassin, Druid, Monk….) and it’s still OD&D. Or add extra races, some of which are traditionally considered to be “monsters”. Still okay. (Heck, early Blackmoor characters were often orcs and vampires and such.)
* Men & Magic listed numbers for character classes up to 18th level or higher, and is still OD&D. So, even though I like low-to-mid-level games best, OD&D can be used for higher level campaigns as well.

Here is the start of a list of general things that sort of define OD&D to me. Not entirely thought out and I’m willing to change my mind or my list. In fact, I’d like to update it once discussion has occurred.
1. OD&D has to have classes. It could have racial classes, I suppose, but feels most natural to me if they are profession classes. Skill systems just don’t feel OD&D-like to me.
2. d20-based rolls feel more OD&D-like than percentile rolls. (Using d6’s isn’t a bad approach, either. In fact, using mostly d6’s is usually better than funky-sided dice, but combat feels best as a d20 most of the time.)
3. Rules in OD&D are a skeleton on which the game is built. Don’t give me the entire muscular system on the skeleton, and don’t take away half the bones, either. I need to be able to see the skeleton.
4. The basic stats shouldn’t be changed. Strength, Intelligence, Wisdom, Constitution, Dexterity, and Charisma. It’s okay to find a new use for a stat, but don’t add or subtract from those six. (I once considered combining INT and WIS into a “mind” stat. I thought it would be more useful and help avoid the “dump” problem, but it wasn’t OD&D.)

Looking for more ideas....

[James Maliszewski]

admin said:

Skill systems just don’t feel OD&D-like to me.

So no Thieves for you, I presume?

[Deleted]

admin said:

Skill systems just don’t feel OD&D-like to me.

I agree. Skill systems are fine, if the entire game system supports them. I just don't feel OD&D does (I don't even really consider it a game system, more like an outline, really). IMO, OD&D is much looser in scope, & adding a huge skill system just takes away from what OD&D is: mechanically simple.

IMC, I've ported over the AD&D2e. Secondary Skills (plus I added "Herbalism"). A player may choose (1) for her/his character at the start of play. I use SS to represent what the PC is really good at outside of dungeon-delving: If your SS is "Woodworking/Carpentry", then your PC can pretty much do anything in that skill reasonably well--no roll needed. If it's something way out there, I'll make them roll an ability check. However, SS in my game are certainly not all-inclusive of what a PC is capable of doing; a player can have their PC try anything they can possibly dream up; I'm the DM, & I'll adjudicate it. Still very simple, but adds a little character "flavor".

I also allow the Thief, & the Monk as PC choices, but I've tweaked them so they suit my taste.

The Thief leads me to Percentile-Dice. Now, being younger than most here on this forum, I started with, & grew up with, both Mentzer D&D & AD&D2e--I'm used to percentile-dice (even if the Thief is the only class that would really use them). I know percentile die really bother a lot of people, but me, not so much. If I had the time, I would try to create an "All d6" version of D&D, but since I don't, I'm not going to bother changing it. Percentile-dice are in (for me at least).

Speaking of Mentzer D&D, I also use the combat system from that edition as well. It's what I'm comfortable with, & it works just fine for us.

As for "racial classes", I like them. But we don't have to call them that: An Elf is a Fighting-Man/Magic-User, a Dwarf is a Fighting-Man, a Halfling is a Fighting-Man, etc. For me, it really comes down to how you percive them & what you call them. IMO, in OD&D, class & "racial class" is one & the same.

Now I could go on & on about campaign-centric house rules on everything under the sun I use, but they're really not up for discussion here. I agree with Finarvyn's assesment at the top of this topic: I need a skeleton, but not the meat. I'm a natural tinkerer, so if you just give me the template, I'll do the rest.

In all, I think you can change a lot about OD&D; but I'm not so sure about adding more die mechanics...

My OD&D games are probably a lot different than most other DM's, but to me it's still OD&D.


Just my version.


[EDIT]: I've actually got some pretty decent ideas about an "all d6" game mechanic for OD&D, but I'm not a math wiz by any stretch of the imagination. I'll start a thread, though. ;D

[jrients]

For me the answer is more procedural than mechanical. Can I still quest for riches and glory in wicked cities, dangerous wildernesses, and mysterious underworlds? Is the majority of fiddly resolution handled by the good judgement of the referee rather than set in stone rules? If both those answers are a resounding "Yes!" then I can accept all sorts of variations from the baseline mechanics.

[ffilz]

For me, there's various levels of OD&D. In a sense, if you are playing with anything more than very minor house rules beyond what's in the books, you are not playing OD&D.

With RPGs, and the focus on creative contribution, which essentially requires doing stuff that's at least not quite in the rules, I think perfection is hard to reach.

That said, I think a lot of people do games a disservice when they claim they are playing XYZ Game but have so many house rules that a new player who thinks they already know XYZ Game will be lost. It's even worse when they complain about XYZ Game being broken, when it is their mistreatment of the game that is broken.

There are some core parts of the game that I think changing renders the game definitely no longer D&D (I leave out the O on purpose, because too loose a definition will include at least the Basic/BX/BECM/RC line of thought as OD&D). Definitely you need classes and levels. I think you do need the d20/AC combat system, though using Chainmail is a supported option (and documented in Men & Magic as the standard system though apparently the authors no longer or never used Chainmail in the context of D&D). I think the game also has to be fundamentally based on going out killing things and taking their treasure to gain power, fame, and fortune.

To this extent, I think skill systems risk dragging the game away from it's core. If you start having characters feel like they should be doing stuff other than killing and taking treasure, the game is on the path to being something different. The thief probably squeaks by since his skills are for the purpose of gaining treasure. The social skills in 3.0 definitely drag the game away (and furthermore, don't actually work).

I do like many of Finarvyn's points. Perhaps one good test is look at the character sheet. Does it look like a D&D character to you? I think adding one or two stats, and perhaps even removing one is ok, but clearly too much change is not. Look at a stack of character sheets. Do you see fighters, magic-users, and clerics? Do you see dwarves, halflings, and elves? If not, it doesn't seem very much like D&D.

I think using specific versions of D&D such as OD&D or AD&D require closer hewing to the line, though saying your variant D&D (with new races and classes) derives from OD&D not AD&D does have meaning.

Frank

[coffee]

One thing that I know Gary discussed on occasion but I don't see listed above is this: In D&D, you give experience for treasure.

Seems like a simple point, but whenever I turn around, someone's taking that out of the game.

It's more D&D to figure out a clever way to avoid the monster and grab the treasure than it is to fight the monster head on and lose half the party in the process.

The treasure is what we're there for. The monsters are just in our way. That's the old school way of doing things, and that's a big thing that has been lost in more recent editions where XP for treasure is not an integral part of the game.

Anyway, that's my two coppers (which are worth 1/25th of an Experience Point, based on the Volume 2 ratio...)

[foster1941]

jrients said:

For me the answer is more procedural than mechanical. Can I still quest for riches and glory in wicked cities, dangerous wildernesses, and mysterious underworlds? Is the majority of fiddly resolution handled by the good judgement of the referee rather than set in stone rules? If both those answers are a resounding "Yes!" then I can accept all sorts of variations from the baseline mechanics.

I agree with this. I'm willing to accept a very wide degree of mechanical divergence (no ability scores, no classes, entirely different combat or magic system, etc.) and still consider the game "D&D" as long as it retains a certain essential feel which is, roughly speaking, an ancient or medieval fantasy, planetary romance, or weird-science-fantasy (as opposed to modern or hard sf) setting, a primary emphasis on exploration-based adventure, and resoluton generally handled on the basis of "what makes sense for this character?" rather than "what makes sense according to the rules?" As long as you ave all of those you can throw out or change every rule in the book and as long as you're still comfortable calling your game D&D I've got no problem with it.

Oh, and FWIW I consider "OD&D" to be a ruleset, not a game. You don't "play OD&D" IMO, rather you "play D&D using the OD&D set." This is distinct from AD&D, which is both a game and a ruleset -- you can "play AD&D" (which, IMO means playing essentially BtB -- ignoring or adding to but not changing the rules in the book) and you can also "play D&D using the AD&D ruleset" (changing the rules around to suit your fancy but still using the AD&D rulebooks as the baseline).

[Finarvyn]

jamesm said:

admin said:

Skill systems just don’t feel OD&D-like to me.

So no Thieves for you, I presume?

I don't count the Thief as a skill system. The abilitys to pick pockets, move silently, and so on are part of the Thief class as much as the ability to turn undead is a property of the Cleric class or the abillity to lay on hands is part of the Paladin class.

To me, a "skill system" is a set of rules where a person gets a number of points and a shopping list of skills to buy from. This tends to allow any character to potentially do any thing and blends the character classes together in a way that I dislike somewhat.

While a class system is similar, it really represents a "skill template" where all of the abilities for the class are pre-determined rather than customized as you go.

I prefer the Greyhawk Supplement method for Thieves where there are standard charts, rather than later editions where a Thief got to pack-and-choose in order to specialize.

(I also prefer stardard magic-users rather than spell schools and specialist wizards, but that's for another post.)

[coffee]

admin said:

(I also prefer stardard magic-users rather than spell schools and specialist wizards, but that's for another post.)

Hear, hear!

[Deleted]

coffee said:

In D&D, you give experience for treasure.

Seems like a simple point, but whenever I turn around, someone's taking that out of the game.

Yep. I think that rule has gone the way of Calling & Mapping in modern RPG's...

[murquhart72]

ALL role-playing games are D&D, some are just more heavily house ruled than others

[geoffrey]

What about OD&D's magic system? Could someone replace it with another magic system, and still have the game be OD&D? Or could someone not even have a magic system in his campaign and still have it be OD&D?

[redpriest]

themattjon said:

ALL role-playing games are D&D, some are just more heavily house ruled than others

Why only go back to D&D? May as well say that all modern war games, including fantasy-based, and therefore D&D are Little Wars.

[redpriest]

geoffrey said:

What about OD&D's magic system? Could someone replace it with another magic system, and still have the game be OD&D? Or could someone not even have a magic system in his campaign and still have it be OD&D?

As long as spell-casters are always inferior fighters when compared to fighting men, then why not something like a magic point system? If the spell points are tied to level, then why not?

[badger2305]

One of the first things that happened after D&D came out was that people started to change it to fit their own ideas. Spell point systems, "klutz" rolls, intricate combat systems, magic tables with d1000 distributions - you name it, people came up with it. What appeared in The Strategic Review and The Dragon was the tip of the iceberg: there were tons of things discussed in fanzines, A&E and The Wild Hunt. To be sure, some of it was crud, but remember Sturgeon's Law.

How much can you change OD&D? Judging by the historical record, a WHOLE LOT. The sad part about it is that we see so little of what people actually did to make it their own game - there's been 30 years of attempts to standardize that have gotten in the way.

To answer this question a different way - I think it remains D&D until such a point as you decide it's a different game. Don't let anybody tell you anything different (in my humble opinion).

[ffilz]

badger2305 said:

To answer this question a different way - I think it remains D&D until such a point as you decide it's a different game. Don't let anybody tell you anything different (in my humble opinion).

I think what it really depends is who you are trying to communicate to, and what you are trying to communicate.

When talking to my mom or my wife, calling whatever I'm playing D&D makes sense, they don't understand all the different games (though I think they are aware there are different games).

If I'm communicating to other gamers (especially potential players) hower, I try and be very careful about what I call my game, and how I describe it. It makes no sense to call my game "D&D" if that will cause them to misunderstand me.

Frank

[vladtolenkov]

Here's what I think of as some of the defining qualities of D&D (all editions):

You kill things and take their stuff.

Your abilities are class based.

You level up.

These are actually a feature of many rpgs but all those games are, on some level, just doing D&D. In my opinion, D&D does this better than just about any other game, but where I feel like the game starts to get away from us is when the goals of the "house rules" stop doing the above. After playing a 3E game several years ago, my gaming group and I realized that D&D works best when you don't try to fight what the game's underlying concept is. Having a wild campaign setting is fine. That's just gravy, but the real meat of the game is doing what the system rewards players for.

One reason I now prefer OD&D is that it feels closest to those underlying goals.

Nick

[vladtolenkov]

For more on my points above see James' thread here:

odd74.proboards76.com/index.cgi?board=philosophy&action=display&thread=1197983788

[jjarvis]

One could indeed "change D&D" a whole bunch and still be playing D&D.

I've taken Holmes edited basic D&D and used TFT talents and skill resolution and it was a hoot. The multi dice ability check works pretty good, base task roll 3,4,5 even six dice vs ability score, roll one less die if it is an area of expertise.

I've dumped MU spells entirely and replaced them with Runequest spells. Everyone got a POW attribute added to the list to deal with this and MU's got bonus Pow points, i left clerics alone in that campaign clerics and MUs were very different indeed.

Hit dice as parry. This one causd a schism and was only used for a couple of weeks. Players didn't have a hit die score. they rolled their hit dice each time hey were hit as a "parry" against the blow, equal or beat the blow no effect don't keep track of the wound. Don't beat the blow and keep track of the damage and suffer a -1 to all saves and attack rolls you make until haled, all 1's and you are knocked out by the blow. You only died when damage exceeded total possible HP. Some players really liked it, others hated it. The ones that hated it, REALLY hated it so it was dumped.

All of the above were still D&D.

[James Maliszewski]

jjarvis said:

All of the above were still D&D.

I personally think it's more accurate to say that "all of the above were based on D&D." At a certain point, if you change enough mechanical features, I'm not sure it's possible to call every variant "D&D" anymore or else the term in meaningless. I don't think, for example, that Empire of the Petal Throne can be called "D&D," even though it's clearly a hack of OD&D. Too many things have changed, both mechanically and conceptually, for me to call it anything more than perhaps "a D&D variant," but I believe there does come a point when you can change too much.

[jjarvis]

jamesm said:

jjarvis said:

All of the above were still D&D.

I personally think it's more accurate to say that "all of the above were based on D&D." At a certain point, if you change enough mechanical features, I'm not sure it's possible to call every variant "D&D" anymore or else the term in meaningless. I don't think, for example, that Empire of the Petal Throne can be called "D&D," even though it's clearly a hack of OD&D. Too many things have changed, both mechanically and conceptually, for me to call it anything more than perhaps "a D&D variant," but I believe there does come a point when you can change too much.

Almost everybody's house ruled campaign is a variant. Doesn't make them "NOTD&D". Even minor changes to procedure have an impact on the game.
Use of published options alone creates a host of permutations; a campaign that uses the man-to-man tables is very different from one that used the straight level based combat charts with d20 roll, the greyhawk to hit modifiers based on armor type alters things as does the revised variable damage system. Use the blackmoor hit location tables and it's certainly a different experience then someone who doesn't use them. All make for different games that are still D&D.

[James Maliszewski]

jjarvis said:

Doesn't make them "NOTD&D".

Perhaps this would make for a good topic in a new thread, but how are you defining "D&D" then? By this logic, almost anything that starts from the mechanical/conceptual basis of OD&D and builds from it could count as "D&D" and that strikes me as odd (no pun intended).

[sham]

Excellent thread with some very valid points. I do think that the topic is being broadened a bit into house rules and interpretations, but I want to get back to what Fin said actually makes these different variations still truly D&D.

1. Class Based Character System
2. d20 for Combat (or Chain Mail Combat)
3. A Foundation of Basic Rules (from Vol.s I-III)
4. The Six Cardinal Character Abilities

That's a very broad and generous list of definitions. I'd say that we are all using these four 'keystones' in our OD&D games.

To delve a bit further:
1. Class vs. Skill based. This is in regard to the Characters themselves and how they progress after gaining experience. This is not to say that certain classes do not have skills, just that Characters use a Class or profession, as opposed to say a CoC style skill-based Character progression.

2. d20 or Chain Mail Combat. Seems straight forward. Once you move into I.C.E. type Combat, for example, you aren't really playing OD&D, it's a hybrid game.

3. Basic Rules. This is somewhat ambiguous. To get to a playable state, in my opinion, the Basic Rules need to be clarified and fleshed out. This does not mean that it's not OD&D. I think what it means is don't define everything, ala AD&D. Also, don't ignore half of the Basic Rules and expect to call it OD&D. More on this later.

4. I don't think there's much, if any, disagreement with this one. To me, 'statless' (not diceless mind you) is not OD&D, or any version of D&D for that matter. If you wanted to distill stats (abilities) down to simple reference or description, I think you are toeing the line between D&D and story mode. The ability scores of OD&D Characters are essentially their DNA codes. I think it's fair to say statless is not OD&D.

I hope I'm reading Fin's intent properly. Even with the above definitions, I think the possibilities of clarification and home brewing are still virtually limitless.

I might be tempted to add a few further definitions, but I'd have to think long and hard about what exactly those would be. Simple aspects that are not only OD&D conventions, but D&D in general, could be included and defined.

This thread did get me thinking about OD&D as played from the persepctive of someone who only owned Vol. I. I'll start a new thread for this, though.

[James Maliszewski]

To clarify my own perspective:

If a mechanical system/concept is either presented unclearly/ambiguously in OD&D or its supplements (e.g. how to handle elven multiclassing) or OD&D or its supplements provide options/alternatives (e.g. combat system), there is scope for house ruling without straying so far from the original intent of the rules. Likewise, if OD&D or its supplements show by example that certain mechanics/concepts are extensible (e.g. character classes), then that's fine too.

However, once you start replacing major mechanical systems/concepts without prior examples in OD&D or its supplements, I feel you're straying too far to call what you're playing "D&D" except in the most positivist sense.

[redpriest]

sham said:

.. I want to get back to what Fin said actually makes these different variations still truly D&D.

1. Class Based Character System
2. d20 for Combat (or Chain Mail Combat)
3. A Foundation of Basic Rules (from Vol.s I-III)
4. The Six Cardinal Character Abilities

...

2. d20 or Chain Mail Combat. Seems straight forward. Once you move into I.C.E. type Combat, for example, you aren't really playing OD&D, it's a hybrid game.

I agree with you on points 1,3 & 4, but slightly disagree with regarding 2. I agree that if you significantly change the combat rules, especially if it's a change toward simulation and complexity and away from abstraction, then you're treading onto thin ice to keep your OD&D feel. However, using a system that maintains the abstract nature of combat, in that, a round reflects a combination of blows and maneuvers rather than a blow-by-blow simulation, could very easily fit into OD&D.

One significant thing to consider when adopting combat rules that are not Chainmail's Man-to-Man or M&M's alternative rules is to ask, "how, if in any way at all, does this change affect non-combat rules?" Say that you have a system that maintains the 10 rounds per turn rule, quickly resolves attacks with just one or two die rolls, allows combatants to fight better as they increase in level, ensures that fighting-men are always better at weapons combat than magic-users and doesn't affect a single rule outside of combat. It's easy to envision such a system being in the spirit of OD&D.

Basically, I don't think it matters if you're rolling 2d6, 1d20 or 1d6 or even d100, as long as it's simple, relatively quick and tactically satisfying, then you're likely still on firm ground.

[badger2305]

jamesm said:

However, once you start replacing major mechanical systems/concepts without prior examples in OD&D or its supplements, I feel you're straying too far to call what you're playing "D&D" except in the most positivist sense.

If I'm not mistaken, what we're engaging in here is somewhere in-between the Slippery Slope argument and what we used to call the Argument of the Beard: "how many hairs are necessary to make a 'beard'?" (and thus suggesting that there is no difference between being clean-shaven and having a beard)

Put another way: how many changes constitute something other than D&D? If you argue that any changes make it not D&D, then you've fallen for the slippery slope fallacy (so to speak) - and we know that's not really true, anyway (I'll come back to that). If you argue that you can change everything and still be D&D, you've grown into the argument of the beard fallacy. But there's a point where we go from playing D&D to playing something else, so we ought not turn "playing D&D" into a meaningless phrase.

Playing D&D also contains a logical paradox that can't be easily resolved, namely that playing "by the book" is not considered to be playing the game appropriately. This logically means that there is no single standard of "appropriate play" - we must instead appeal to a kind of meta-play (which is why I suggest that personal intent is important). In fact, for every individual rule in the game, it is possible to suggest a variant of some kind that does not change the game from D&D. But a sufficient aggregation of rules changes eventually constitutes a different game.

Let's take an example: Runequest. To get to RQ from D&D, you need to change a bunch of things; add a POW characteristic, base magic off of that, completely swap out the religious system from a fuzzy Christian-inspired one to a polytheistic cult system, change allowed PC races, and come out with an entirely different combat system, add a skill system, and rewrite the experience system. With the benefit of hindsight, we can see that at some point, there was the intention of writing a different game, but RQ's writers started off playing D&D. Nobody would suggest that playing RQ is the same as playing D&D, but when did the changes cross over from the old game to the new one? That's less clear.

So I would suggest that it's not just quantity of changes, and not just quality of those changes. It's a combination of both. In keeping with the magical nature of gaming, I'd suggest that it is a matter of intent, as well. If you intend to play "D&D" insofar as D&D is symbolic of a certain kind of game, that matters. (Case in point: a friend of mine right now is running an Exalted game, but commented wryly that a player in that game was "playing D&D" )

On a practical level, I would suggest there are broad categories of D&D game play that might help describe what's going on:

• "Playing D&D" - this is game play that involves relatively few house rules (but remember that some house rules are not only possible but are to be expected). The idea here is to play within the spirit (not the letter) of the game rules.
  
• "Playing a D&D variant" - this is game play that is clearly based on D&D but may involve significant changes to the rules. One example of this would be Empire of the Petal Throne; the rules changes themselves are actually rather minor, but the emphasis on setting creates a variant form of the game. Another would be Arduin, where setting is lightly sketched out, but the rules variations go for miles.
  
• "Playing a different game" - at some point, a game moves from being D&D to being something else. Some of this involves specific rules differences, and some of it involves envisioning a different game entirely (and we're back to intent).
  

Since the boundaries between these difference categories are not easily defined, any attempt to come up with specific "game elements" must be couched in an argument of meta-play; rather than mechanical elements that cannot be touched, the items mentioned are symbolic of what makes the game "D&D" as we collectively understand it. So it isn't a matter of this piece or that piece, but the various premises that create a particular kind of whole (which suggests that we're dangerously close to syllogistic fallacies, but we're stuck with it).

There, that's clear, right?

[James Maliszewski]

badger2305 said:

There, that's clear, right?

Mostly, it is. I think you've pretty well laid out the pitfalls and perils of this particular discussion. At the same time, you're not dismissing the notion that it is possible to house rule yourself out of D&D entirely, whatever your original intent. The notion of aggregation is key here; it's rarely any single house rule that makes a game no longer qualify as "D&D," but an accumulation of them.

That said, I do think there are some sine qua non elements, the loss of which immediately make a game "not D&D" anymore, because we're talking about bedrock mechanical/conceptual things, such as character classes, for example.

[badger2305]

jamesm said:

That said, I do think there are some sine qua non elements, the loss of which immediately make a game "not D&D" anymore, because we're talking about bedrock mechanical/conceptual things, such as character classes, for example.

Right. Another conceptual element would be the idea of experience leading to significant advancement and growth of ability. Traveller, for example, starts with the exact opposite premise: you must be experienced before you go adventuring.

[James Maliszewski]

badger2305 said:

Right. Another conceptual element would be the idea of experience leading to significant advancement and growth of ability. Traveller, for example, starts with the exact opposite premise: you must be experienced before you go adventuring.

Correct. I suspect, if one were objective about it, we'd soon find that there are in fact quite a large number of mechanical/conceptual elements in OD&D that are non-negotiables.

[ffilz]

Given that it's essentially impossible to play "by the book" (if nothing else because part of what it means to be an RPG is to allow creative contribution to the game), it does make it very hard to say "that's D&D" and "that's not D&D."

One test that can be simple is to try and verbally describe your game. If at some point, you say, "aw, heck, here, read this," then you almost certainly have "not D&D." (assuming the reading material isn't just, for example, a different saving throw chart - which could be very hard to describe how it's different verbally, but very easy to see on paper, but other than the set of values chosen, is used the same way, so yes, it probably does change the game, probably by subtly changing the role of one of the character classes, but the the game is still going to play out the same way from a higher vantage point).

Badger's point about the beard is a good one. It's useless to make an argument that says "all games are D&D." We already have a way to express that "they're playing a game" or "they're playing a wargame" or "they're playing an RPG." But it's also a good point to note that just because the only books lying on the table are "D&D" doesn't necessarily make the game D&D. Nor does the fact that there is also a Traveler book on the table in addition mean the game is no longer D&D (perhaps the Traveler book is there to provide the GM with some descriptions of some new monsters - nothing more, he also has a sheet of paper with the HD, AC, attacks, etc. for those monsters - i.e. they are D&D monsters, inspired by Traveler).

There is also a good point of looking at the way a player is playing and identify their play as fitting D&D better than Traveler. But that's not characterizing the game (and it may even be wrong, perhaps their play is inspired by Tunnels & Trolls more than D&D, but in comparison to Traveler, and without familiarity with T&T, it looks like D&D play).

Sometimes an apple is an apple, other times it's an orange, sometimes it's a pie, sometimes it's a turnover, sometimes it's a crisp or cobbler, and sometimes you can't tell if the mush is an apple, an orange, some of both, or something else entirely different (perhaps not even a fruit).

Frank