What is the minimum viable D&D?

From this post: B/X is close to a "minimum viable D&D", where every mechanic has a clear purpose. It still has some redundant parts (and some missing parts, IMO; I think every D&D should have a ranger or other way to meaningfully raise your chances at exploring the wilderness), but it does a great job overall. 

So, the "minimum viable D&D" is a form of D&D that includes every mechanic D&D to play a D&D game, and little else. In theory, you could play a D&D module using Risus and a few d6, but that is not what I'm talking about. What I'm trying to identify is the very essence of D&D - things that must be included in the core books of every edition to give you a D&D feel. Things that people came to expect from D&D. For example:

- Six ability scores.

- Three or four races.

- Three or four classes.

- Saving throws.

- AC and HP.

- XP and levels.

- Spells.

- Encumbrance.

- Equipment, including multiple weapons with differences.

- Treasure, including magic items.

- Rules for combat, interaction (reaction, morale, languages, etc.), dungeon exploration, and wilderness exploration.

- Monsters and HD.

Another thing to consider is how these things relate to one another. For example, your ability scores and your class affect your HP and saving throws. Let's call them "ties"; there are direct ties between abilities and AC. A mechanic that is tied to no other mechanic can be called "untied".

This is the PHB stuff. You also need monsters, GM advice, etc. I'll focus on PHB stuff that allows you to play existing OSR adventures.

Notice that there are some parts in B/X that I think are NOT strictly needed. They are add-ons to the essential parts. For example:

- Multiple specific saving throws.

- Multiple XP tables.

- Spell slots.

Let's tackle these and other issues to see what else is MVD&D.

This is how AI sees D&D covers.

Ability scores

It is certainly possible to play D&D without them (see The Searchers of the Unknown RPG). Conversely, you could play using mostly ability scores with the exclusion of other aspects (see Knave, The Black Hack, etc.).

However, they have been in every edition, and I think they are now part of the D&D experience.

Thieves

Thieves fall into a gray area. Their skills are deeply tied to the dungeon setting, but they are mostly add-ons, with the exception of hear noise and find trap (that create a connection between class and those actives... that would otherwise be affected by race). Open locks is not necessary since other ways to deal with doors (force doors, knock) already exist. Move silently and hide in shadow do not tie directly to existing mechanics such as surprise. Etc.

Overall, I think thieves are MVD&D at least in feel. They've been in every edition since Holmes. Notice that thief and paladin were officially born simultaneously in OD&D Supplement I: Greyhawk (0e), but the paladin is somewhat of an hybrid class, while the thief is very much its own thing.

Skills (and rangers)

Non-thief skills are not strictly necessary either. However, they are immensely useful to tie existing mechanics. 

For example, in B/X, there are rules for hunting and foraging, but no way to be better at either. Same for surprise and initiative, which become "missing ties" if there is no other mechanic to interact with them.

You could add this missing ties by allowing your Wisdom bonus to affect some of these rolls, for example, without needing skills. Skills are there to make these things a lot easier, IMO.

At minimum, we'd have thief skills, plus maybe nature and persuasion (to affect hirelings, morale, reactions, etc.) - and also make a decent/obvious tie between these three things:

- Hide/move silently.

- Surprise.

- Backstab.

And, if we are looking for the minimum, one single skill mechanic (percentages, x-in-6, 1d20, etc.) will suffice.

Clerics and paladins

These are hybrid classes; fighters that cast spells, especially for the paladin. There is also turn undead, which is a curious thing. Some undead are usually not tied to morale rules, making them especially dangerous. So, this might be a nice addition, but definitely an add-on.

There is a good case to be made for the cleric, but also against it. Overall, I'd say it gets the MVD&D status for being in every edition. Not so for the paladin.

Bards and other classes

I feel that a class that is especially adept at social interaction (the "face" of the party) would be useful, although Charisma can play that part independently. The thing is, the cleric is not necessarily filling this niche either - instead, it becomes a mix of thief, MU, and sometimes fighter. I am not a fan of the bard in general and I think a warlord would be more appropriate - with ties to morale, loyalty, hirelings, etc. 

Likewise, I don't find druids, barbarians, and monks strictly necessary either. They are cool, but not the minimum. Every B/X player is doing fine without them.

Multiple specific saving throws

Every edition has multiple saving throws, but they work differently. You have the original categories (wands, spells, death, etc.), the 3e/4e division (reflex/fortitude/will), the 5e model (tied to each ability score), etc. 

You must have saving throws, but which saves are a matter of choice. 

Again, since we are looking for the minimum, one single save will suffice.

I'll admit having Mind/Body STs is incredibly alluring to me for some reason.

Multiple XP tables

You could have a D&D game without XP - I'm playing such a campaign right now - but XP is in every edition and has ties to class, level, treasure, etc. Part of the minimum, IMO. And, again, there is no reason to have more than one table if we are looking for the minimum.

Spell slots

This is a curious mechanic that doesn't really interact with ability scores (except that in some versions you get more slots). 

I'd say that spells are very much mandatory in D&D, but spell slots are not (4e didn't have them IIRC; 3e and 5e has optional spell points and other methods).

You could tie them to existing mechanics (such as saving throws or hit points) instead.

That might be a personal preference - see below.

Different Hit Dice

I used to dislike different HD because I thought it was somewhat redundant with Constitution. Now, I'm a bit on the fence - the fighter has few features and we should be very careful when taking them away. In any case, HD is needed, even if all classes have the same HD (and some have more HP due to Constitution).

In addition, I really like giving bigger hit dice to big monsters (such as giants etc.), which is something common in modern D&D and even considered by Gygax IIRC (if he had been responsible for 2e).

In conclusion....

As you know, Dark Fantasy Basic was my first attempt at re-writing B/X to my liking. I'm still playing it with a few modifications (currently running a sandbox campaign). But DFB is not exactly minimalist. It contains lots stuff that I find cool and not strictly necessary. 

I might write a minimalist D&D one day - the beta version is here. This is only half-baked ATM; I'm having a hard time choosing a single mechanic for combat, spells and skills. 

Spells are the main problem, since everything else can use Target 20. 

And I haven't been a fan of Vancian casting for a while - I especially dislike spell level that do not correspond with character level!

But I'm very close to simply ignoring the existing spell system and creating something entirely new (and NOT necessarily compatible, unlike my earlier attempts) - maybe DCC-based, and much simpler than spell levels, slots, etc. 

Stay tuned for that!