Pre-Publication D&D and Target 20

Jon Peterson has, as usual, posted an extremely interesting snippet on his Playing at the World blog, for the pre-publication draft of D&D now known as the "Guidon Draft". Specifically it's the following:

Jon presents this in the context of the question, "Why did armor class descend from 9 to 2?". Which is of particular interest because the prior work, Chainmail, did actually have ascending AC, so one wonders what motivated the flip. And here we have an answer: The initial concept for the system was not table-based, but rather, a simple formula in which one could subtract the AC from 20 to see the target number one needed to hit. For higher-level fighters, the chance would increase by one pip per level.

Jon kindly links back to our blog here as he includes the observation, "in its relentless quest to perfect combat systems, the OSR has previously recognized this as the rough algorithm behind the original attack matrices". And one of the commenters on his thread writes, "How cool to see Target 20 in the pre-publication D&D." We are happy to agree.

Of course, the system above is based on subtraction, whereas for Target 20 we think it's easier for players to use the equivalent addition algorithm (roll d20, add fighter level and target AC, and check if the result is 20 or more). Also note that there's at least a one-point slippage in the exact system above; there's an 18 hand-overwrritten with a 19, a correction which didn't transfer to the associated combat table. This is broadly in synch with the one or two pip error we accept with Target 20 for the sake of simplicity. Also note the draft statement 2 - 20 = 18, and that we can imagine two different perspectives in which either 18 or 19 is related to the "90%" figure. As Jon said on the OD&D Discussions forum, "These things were... loosely reckoned".


  1. I love seeing this kind of gaming archaeology. Knowing this it's easy to see how things progressed in game development. How very cool.

  2. Was AC 0 ever intended to be a hard limit, or has -AC pretty much always been with us?
    Descending AC with a limit at 0 makes sense, "but this goes up to -eleven..."

    1. From the brief description of magical armor in Monsters & Treasure, it's fairly obvious that magical "pluses" from armor and shields did not stack with one another.

      "ARMOR: Armor proper subtracts its bonus from the hit dice of the opponents of its wearer. If the shield's bonus is greater than that of the armor there is a one-third chance that the blow will be caught by the shield, thus giving the additional subtraction."

      In fact, the wording is debatable so there's even a valid argument that magical armor bonuses may have, at one point, not even interacted with AC directly. There's a very reasonable interpretation that "subtracts its bonus from the hit dice of the opponents" refers to the "Monster's Dice #" on Attack Matrix II: Monsters Attacking. E.g., a 4+1 HD ogre attacking a man in Armor +3 would be reduced to the attack column of a 1+1 HD creature vs. AC 2.

      However, the trouble is that this may not have accurately reflected the way either Gygax or Arneson played at the time. For example, the three little brown books contain many references to using Chainmail combat tables to resolve melee, but throughout the years whenever he was asked about it, Gygax consistently maintained that his group had been using the d20-based "alternative" combat system more or less since the beginning.

    2. I pretty much agree with what Daniel Wakefield is saying above.

      I could actually imagine that at first publication AC 2 (top of the table) was effectively the "hard limit". That's the best AC for any monster, and it doesn't seem like anything in the game at that point directly affected one's AC number in any way (no Dexterity modifier, that now-very-weird rule for magic armor in Vol-2, magic rings of protection work the same, etc.)

      Contrast with Greyhawk p. 14-15 that presents a table with the modern understanding (and zero, negative ACs in print for I think the first time).

  3. This is what Thac0 is....
    It is explained exactly like this in 2nd edition ad&d.
    You subtract AC from thac0 to find the number you need to roll for a hit.

    I prefer this to target20 or ascending ac as i want to know what to roll before i roll. "Post roll math" feel anticlimactic to me. I want to roll the die and hold my breath and wait for the actual dieroll to decide the result instead of a post roll formula. This of course a matter of taste.

    1. It's a reasonable point, honestly. Counterargument is that maybe players shouldn't know the enemy AC (?). I do find that as DM I'll do the subtraction before declaring & rolling dice for a mob of monster attacks. Sometimes I wish that D&D used a roll-under-accuracy method, so the overall target value would be an additive level + strength + AC.