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As far as I can see, one of the watershed changes with D&D 3e, which has survived down into 5e, was the replacement of a plethora of different systems for determining success to a d20-roll-over mechanic: Armor Class for attacks and Difficulty Class for everything else. While D&D Armor Class has a pedigree going back to Chainmail (though it switched direction back and forth), Difficulty Class seems to be a purely 3e innovation.

My question is: Who in particular can be credited with the invention of D&D Difficulty Class? Are there any comments from around the creation of 3e to suggest which person or people came up with this radical streamlining of tests of non-combat abilities in D&D?

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  • \$\begingroup\$ Though you compare it to armor class as if they are the same thing, armor class is not the same as it used to be. It has changed to follow the new d20 system the same as everything else. The only similarities to old armor class and attack rolls to new are 1) It used a d20, and 2) the words "armor class" were involved. The roll itself, using your THAC0 ("To-hit-armor-class-zero") chart, was much different. \$\endgroup\$ – Aaron Jul 13 '17 at 18:29
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Jonathan Tweet invented 3e’s core mechanic and its DC concept

Difficulty Class did indeed make its D&D debut in 3e,1 and has been traced by Shannon Appelcline's historical work directly to Jonathan Tweet, lead designer of Dungeons & Dragons 3rd Edition.

Initial 3e work began without a design vision shortly after WotC acquired TSR, and CEO Peter Adkison stepped in to give an overall goal and philosophy and to install Tweet as the lead designer. Tweet proceeded to establish the core mechanic — and hence Difficulty Class — as “die + bonus”, a core mechanic initially developed by Tweet for and appearing in the first edition of Ars Magica.

Appelcline in his own words, from Designers & Dragons: The '90s, pages 155–6:

Peter Adkison — who had previously left third edition design work to Bill Slavicsek and a group of ex-TSR employees — decided to write a basic philosophy for the new edition that would guide future work. […] He called for a new game that would keep the feel of the original D&D rules, but would throw out things that didn't make sense. He also named Wizards of the Coast RPG head Jonathan Tweet as the new leader of the third edition project. […]

Using lessons learned from Alternity, it offered a unified task-resolution system. The core mechanics were drawn from the “die + bonus” skill system that Tweet had created for Ars Magica (1987) over a decade before. Its main difference from Alternity was that it was a roll-over system rather than roll-under: skills were added to die rolls and compared against either target numbers or opposed skills.

Though the inspiration for using a unified mechanic came from Alternity, that system doesn't feature DCs or an equivalent, instead having you roll under your own ability or skill scores to achieve a success. Flipping it around to use his “die + bonus” roll-over mechanic required a target number external to the character, as in Ars Magica — and by Tweet importing that design, thus did Difficulty Class enter into D&D's design DNA in 1999 and lives on to this day.

As an editorial note, I find it interesting that today it is Difficulty Class that we consider characteristic of this task resolution system. At the time, Tweet's own conception of the mechanic's innovation wasn't DC, but the idea that situational effects were reflected in a unified modifier (rather than alternative die sizes or modifiers on your stats, mixed with a few low-value straight roll bonuses). DC was just a necessary invention to accommodate that design. So although DC is the most visible characteristic of this core mechanic, this is somewhat backwards historically, as the original heart of the mechanic lies elsewhere.

Appelcline talks about this innovation within the RPG design context of the late 1980s — and credits it as an innovation of Tweet's — more in the 80s volume of Designers & Dragons on page 307, in the history of the publisher Lion Rampant2:

One of the notable innovations of Ars Magica was Jonathan Tweet's target-based skill roll (or, “die + bonus” as Tweet calls it). Early RPG systems had mostly used simple roll-under mechanics (e.g., percentages) or roll-over mechanics (e.g., THAC0). They were ultimately limited in range by a die roll, and initially offered no opportunities for comparison contests. These early systems were now being innovated by games like Pendragon that used a roll-under-and-highest-roll-wins mechanic for competitions.

Tweet felt the need to have contests, modifiable rolls, and results that weren't bound by die size. He introduced “die + bonus” and targets in Ars Magica. The idea was a wave that hit the industry at the time; it can also be found in Bard Games' Talislanta (1987) and R. Talsorian's Teenagers from Outer Space (1987) and Cyberpunk (1988).

As obvious as design as it seems like today to have an uncapped bonus added to a die roll, it was something new under the sun in RPG design at the time. We can thank Jonathan Tweet for the idea of rolling and adding a small-or-large modifier in RPG mechanics, and the Difficulty Class that fell out of that design goal.


1. Though D&D has had “target numbers” in its DNA since the beginning, that's unavoidable in most dice systems — you need a number that separates failure and success. The particular core mechanic that made DC prominent and explicit was indeed new in 3e.

2. Lion Rampant was founded by Jonathan Tweet and Mark Rein•Hagen (of Vampire: the Masquerade fame) to design and publish Ars Magica, “a game that did wizards right” (qv. Appelcline). While Rein•Hagen was the creative visionary, Tweet was the meticulous mechanics designer.

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    \$\begingroup\$ Amazing answer! I can't imagine it will be bettered, but I won't accept it yet in order to not discourage others from trying. \$\endgroup\$ – harlandski Jul 2 '17 at 1:15
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    \$\begingroup\$ Die + adds Vs target was in Traveller published in 1977. It's at least a decade older than this answer suggests. \$\endgroup\$ – Jack Aidley Jul 2 '17 at 17:02
  • \$\begingroup\$ @JackAidley It was, yes, but for whatever reason neither Tweet nor Appelcline considered Traveller to be a precedent (and at least Appelcline was certainly familiar with Traveller when he wrote Designers). A form of die + add vs. TN was also in D&D (1974) in the sense that magic swords' bonuses were added to your to-hit roll vs. a TN from a table — I can only speculate that neither met the threshold either man was setting. (Perhaps Traveller's core mechanic is insufficiently open-ended or insufficiently consistent? oD&D's certainly is!) \$\endgroup\$ – SevenSidedDie Jul 2 '17 at 18:27
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The previous answers are incorrect. John Sapienza, Jr. in Different Worlds magazine issues 6-7 (1980), introduced the "Vardy Combat System", which is, verbatim, the combat system (d20 + bonuses vs AC) later used by Tweet and Cook in D&D 3rd edition (2000).

The concept was expanded to include skills, saving throws, everything else, but the basic mechanic of d20+modifiers vs target number existed for 20 years before that game was released.

And the mere idea that a descending AC system is incapable of degree of success comparison for contest rolls is the reason I dismiss Applecline as uninformed.

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  • \$\begingroup\$ I have edited the post to follow our code of conduct. \$\endgroup\$ – SevenSidedDie Jul 9 '17 at 6:14
  • \$\begingroup\$ Any plan to edit the rest of the posts to reflect them being incorrect? If you are going to present Applecline as an undisputed authority on game design, you need to be prepared for the idea that some of us disagree. \$\endgroup\$ – Darva Shriver Jul 9 '17 at 6:23
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    \$\begingroup\$ No, disagreeing is fine and normal here. Content adjustment isn't a mod's job unless it actually breaks any of the non-negotiable rules, in this case our Be Nice rule that says we don't call anyone (on or off site) names. Content disputes are fine, if conducted civilly. \$\endgroup\$ – SevenSidedDie Jul 9 '17 at 6:28
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    \$\begingroup\$ While the d20 System, obviously, has as its core mechanic rolling a d20 with a high result being better than a low result, I thought the combat system's other innovations—like, for instance, eliminating facing in tactical combat, threatened areas, being able to attack then move (unlike Champions!), applying radii directly to square grids, and even the use of 5-ft. squares (like in V&V!)—were much more challenging of the status quo at the time. To me, anyway, bringing together all those other elements under one umbrella makes high-roll-good seem almost an afterthought. \$\endgroup\$ – Hey I Can Chan Jul 9 '17 at 20:33
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    \$\begingroup\$ "Content adjustment isn't a mod's job" Mod or not, you were the original post author I was correcting. Not disagreeing with, correcting. You erroneously credited Tweet for Sapienza's work. \$\endgroup\$ – Darva Shriver Jul 11 '17 at 6:26
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The encyclopedic citation from SevenSidedDie is indisputable. I would just like to add that I asked the creators directly. This is what Monte Cook told me when I asked the origins of the Difficulty Class System, via twitter.

The whole game was a group effort. Designed by Jonathan Tweet, Skip Williams, and myself.

Now, Skip Williams, in Dungeon Master Option: High Level Campaigns (1995), created the True Dweomer System, which introduced a die roll challenged by a cumulative difficulty total of modifiers, that theoretically had no upper limit. This was the foundation for both the 3e Epic level Handbook Seeds Difficulty table, and the later 3.5e Unearthed Arcana Incantations (p. 174) Rules. Skip Williams in interviews argued on behalf of a rules intense game so to as to avoid having so much confusion among players & DMs that he got swamped writing Sage advice columns.He disagreed fundamentally with the rules light - heavy DM interpretation philosophy Gygax espoused, which is probably why you see so many tiny cumulative and multiplicative modifiers in his works.

Here's a quote from DM: High Level

A spell's final difficulty number is usually the same as its adjusted difficulty. When the caster has spent the required preparation time, roll 1d100. If the number rolled is equal to or higher than the spell's final difficulty number, preparations are complete... Once preparations are successfully concluded, the character s free to cast the spell. (DM Option: High Level, p.133, 1995)

Modifiers included things like spell components, higher caster level, and long preparation times (days or even years). For example, Kolin's Undead Legion had a Difficulty of 325, reduced to a Final Difficulty of 45 with modifiers.

To my knowledge, considering the existing rules of AD&D used negatives, low rolling proficiency checks, THAC0, and critical success = Natural 1; this was the first example of a die roll meant to be high vs. a cumulative modifier that also was meant to be high as the difficulty increased, rather than decreased (like -10 AC or trying to make a Ride check by rolling under your dexterity), although here the die roll is a % and adds no bonuses Per Se, most of the Difficulty adjustments were listed as negative modifiers and could have just as easily been added to the d100. (i.e., caster level + 5 (if specialist) + 1d100).

So the dice + bonuses/modifiers vs. Difficulty Class concept was there, but the Ars Magica (1st edition from 1989) reference does sound earlier, where they rolled a d10 + modifiers.

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