Why Did Armor Class Descend from 9 to 2?

One of the great riddles that has vexed D&D players for generations is this: why did armor class in original D&D descend from 9 to 2 instead of increasing as it gets better? The answer is spelled out in the first draft of D&D: if you were a first-level fighter rolling to hit, the number you needed was equivalent to 20 minus the armor class of your target. To hit AC 2, you needed an 18, to hit AC 3, a 17, and so on. Armor class descended to make it easy enough to calculate your needed roll that you wouldn't even have to consult a table. Unfortunately, the published D&D game broke this algorithm, which has obscured the motivation for descending armor class ever since.

A Forgotten Variant: The Observers Book of Monsters

Sometimes chunks of gaming history come down to us without much context. That is the case with today's specimen, a crude British booklet "collected and compiled" by Chris Bursey called The Observers Book of Monsters. It contains a blend of new and recycled creatures for Dungeons & Dragons, of an apparently early date, but gives no direct indication of when or how it came to be made. But thanks to the invaluable fossil record of fanzines, we can connect a few dots and shed some light on its production.

Missile Fire in Chainmail, courtesy of Charles Sweet

When it comes to unearthing the influences behind Chainmail (1971), Gary Gygax and Jeff Perren's medieval rules with a fantasy twist, you need to cast a wide net. Many authors (myself included) have been guilty of singling out Tony Bath as the primary influence behind the mass combat rules in Chainmail, but that has always been something of an oversimplification. There are elements of Perren's medieval rules which drew directly from a 1957 archery system proposed by Charles Sweet, one that recurs in the Lake Geneva Tactical Studies Association (LGTSA) rules up to the publication of Chainmail. A clear connection can be observed in the values in this chart from Sweet's rules, originally published in War Game Digest Vol. 1 No. 4.