A unified d20 mechanic, old school style

One of the things that attracted me to 3.x edition D&D was its (seemingly) streamlined core d20 mechanic. Rather than cobbling together a mish-mash of different dice mechanics for different aspects of the game, 3.x pulled almost everything together under the umbrella of one simple system: roll a d20 and add in all relevant modifiers, hoping to beat a target number that rested on an ascending scale of difficulty.

I liked this a lot. The  old system — especially  the descending Armor Class combat rules, where lower ACs were better, and a positive bonus  to your AC thus lowered it rather than raising it — had never made much sense to me. Sure, I used it for years, and became quite adept at calculating THAC0 scores on the fly. But intuitively, it never bought into it. Every time I tried to explain it to a new player, I’d see my own confused first-time player self looking back at me. And I’d wince inside, just a little.

So, the new-fangled ascending AC system was a big hit with me. I daresay, in retrospect, it was the main selling point of the new edition. And to be fair, I still think it’s pretty good.

But, as I’ve drifted back into an old-school mentality, I’ve wondered what an old-school version of the unified d20 mechanic that incorporates descending ACs (and other difficulty numbers) would look like. Two of the old-school games I’ll be reviewing in the near future — Kevin Crawford’s Spears Of The Dawn and Stars Without Number — give a glimpse.

It works like this: players roll a d20 + modifiers (including an ascending class & level-based “to hit” rating) + the target’s Armor Class, hoping to score a total of 20 or higher. It’s an elegant way to maintain the old school’s descending AC scheme without creating too much confusion, or requiring new players to consult a table of values or calculate their THAC0. Lower Armor Classes, when added to your roll, simply make it harder to score a 20. The really good ACs (the negatives, that is) would actually subtract from your total.

My hat’s off to Crawford here.

But Crawford’s games only use this mechanic for combat, and rely on other types of dice for other systems (notably, d6s for skills). But it could be used in a more across-the-board kind of way.

I’d use it for thief skill checks, saving throws, undead turning attempts, etc., as well as for combat. Every challenge would be rated from 9 (the easiest) to -9 (the hardest). The goal, as above, would be to score a total of 20 or higher on a roll of 1d20 + modifiers + challenge rating. New players, or players whose only exposure to D&D has been 3.x or 4th edition, would feel right at home using a d20 for pretty much everything except rolling damage. No more worrying about which combat or saving throw table to consult.

I’m giving some thought to homebrewing a system similar to this for use in converting my current 3.x game of animal PCs to a more old-school rules set. If it plays well, I could write it up and release it for public use. Of course, I’d make a few changes to avoid ripping off Mr. Crawford too much. But the rules are open content…


About thebobgoblin

It's true. Birds are dinosaurs.
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3 Responses to A unified d20 mechanic, old school style

  1. Coyotecyb says:

    i would be interested to try this out as i still enjoyed the descending AC just because when you were 12 or 13 it was pretty cool to say “I have a -6 AC, come at me bro!” (especially when you were a mage!) It seems as if there are so many new systems now that i get almost kainotophobic and want to revert back to the rules i knew when i was that age. I STILL don’t feel proficient with the 3.5 rules and have been using them for years!
    On another note maybe one of your upcoming blogs can be about gaming mechanics of the 80’s 90’s and early 00’s in respect to the actual players and how we have incorporated all the new technology of the past decade into the game … or how it affects gaming …

  2. I’ll note that the attack roll targeting 20 was something I first saw over at Delta’s D&D Hotspot back in 2009. I liked it so much I used it for SWN, since I wanted to keep descending AC for easier back-compatibility with old TSR material. As for changing things for change’s sake, feel to lift whatever mechanics or systems you like from the games for your own- no retrocloner alive is in a position to quibble about that.

    • thebobgoblin says:

      Thanks for stopping by and commenting, Kevin. I’m a big fan of your work on SWN and SOTD, so the visit is an honor.

      I’ve actually written up a brief rules engine along these lines, and will take them for a test drive in the near future. The more I think about it, the more interested I’m becoming in writing — and possibly self-publishing — a stand-alone retroclone devoted to animal fantasy.

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