Defining Characteristics of a d20 RPG?

[Electric Wizard]

What are the elements of a d20 based RPG which are not found in other games systems (or not stressed as a primary mechanics-lever)?

I think the first thing has to be well defined levels in a Class. As opposed to Attribute increases in, for example, Shadowrun which is an Attribute & Skill based mechanics system.

Any others?

yes:

I didn't mention d20's so *YOU* can bring it up.

[Haladir]

There are plenty of game systems where the resolution system involves rolling a d20, but not all of them would normally be considered "d20 system" RPGs.

Digression about the D20 System license:

First of all, "D20 System" was a separate game license that Wizards of the Coast used alongside 3rd Edition Dungeons & Dragons, from 2000 through 2008. "D20 System" games used the Open Game License (OGL), and had a few other restrictions. Systems that complied with the system were entitled to use a "d20 System" logo on their covers. An example of a "d20 System" game that wasn't explicitly compatible with D&D 3rd Edition was WotC's own d20 Modern RPG.

All licensed d20 System games used the OGL, but not all games that used the OGL were entitled to use the "d20 System" license. WotC ended the "d20 System" license when they released 4th Edition D&D.

Still, "d20 system" is used in most TTRPG circles to refer to games that use the Open Game License.
* * * *

I think what makes a "d20 system" game is the core resolution mechanic: When chance is a factor and the result isn't a foregone conclusion, roll 1d20 and add one or more relevant modifiers. If the total is equal to or greater than a target number set by the GM, then the action is a success. Otherwise, it's a failure.

In other words, you always want to roll high to succeed.

I don't think that character class is necessarily a defining characteristic of "d20 system" games. I would cite Mutants and Masterminds as a prime example of a "d20 system" game that does not have character class or level.

TTRPGs that clearly are "d20 system" games would include...
Dungeons & Dragons 3rd and later editions. (Yes, I'm including 4e, even though it didn't use the OGL.)
Pathfinder 1e & 2e
Starfinder
Castles & Crusades
13th Age
d20 Modern
True 20
Various D&D "retro-clones," such as OSRIC, Swords & Wizardry, Dark Dungeons, Labyrinth Lord, etc.
Mutants & Masterminds
Spycraft RPG
Star Wars Role-Playing Game (WotC, 2000-2010)

...and a LOT of others.

There are other TTRPGs that use a d20 as its resolution mechanic, but do not use the core OGL game mechanic. Consequently, I would not consider these "d20 system" games.

For example, there are "roll under" systems, where a PC rolls against a relevant attribute value (never higher than 20), and success is determined by rolling under that value. In "roll under" systems, there's generally no modifier, and you want to roll low to succeed. I pose that these game systems aren't "d20 system" games, as the mechanic is sufficiently different. Games that use a d20 as part of their resolution mechanic, but don't use "roll 1d20, add modifier, beat a target" resolution include a number of second-wave "Old School Revival" (OSR) RPGs that stepped farther away from D&D while keeping other OSR principles. Some examples are: The Black Hack, Blueholme, Into the Odd, SEACAT, Esoteric Enterprises and others.

[thejeff]

Is there a reason D&D retro-clones fit, but AD&D/Basic themselves don't?

THACO is weird, but mechanically equivalent to the core mechanic, unless I'm missing something.

Obviously, they're not in the sense of used the D20 License or the OGL, but mechanically? I'd have to think if there were other early pre-3.0/OGL games that used a d20 roll high system.

(Rolemaster, IIRC, used percentage dice, but essentially did so by multiplying everything in D&D by 5.)

[Haladir]

Earlier versions of D&D are in the "d20" family, but they're not "d20 system." They're a lot messier, in that they don't have an internally-consistent/unified resolution mechanic. Attack rolls are resolved via a table look-up that didn't always scale consistently. (This was simplified to a THAC0 formula in B/X and AD&D 2e, but still used tables innthe books.) Saving throws are also a table look-up that doesn't scale evenly. Ability checks are all "roll under your score" (i.e. roll low) until 3e.

Most of the OSR retro-clones weren't exact reproductions of the D&D edition they emulated: The designers usually recodified the rules to fix inconsistencies into a unified resolution mechanic, per the OGL.