Armor class 2 originally meant two increments of 1-20, i.e. a 10% chance of being hit by a first level fighter!
Originally shared by Jon Peterson
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.
I remember Tim Kask on DF saying that the AC9 to 2 progression was originally adapted from a naval war game.
ReplyDeleteAnother aspect that may or may not have been an influence on AC in OD&D is that the armor "classes" were always a static value, not a moveable target number - so yes, the comments discussion on Jon's blog hit that nail on the head - you could just label the columns by any designation aside from the "confusing" AC values of 9 through 2.
By "static value", I came to understand that AC 9 always meant "no armor", even if the target had a +1 Ring of Protection. AC 2 always meant Plate Mail & Shield, even if the target had +3 Platemail and a +5 shield. The attacker attacked the target's AC value, and modified (by way of negative values) the attack roll according to the magical bonuses.
Of course, to complicate matters, if the attacker had a magic weapon, the attack roll was positively modified.
It made sense to lessen the amount of mathematical steps by making the AC value a target number, so not only was a target with +2 chainmail & a +3 shield have an AC 0, so was a target with platemail & a +2 shield.
It made even more mathematical sense to do away with AC values in later editions, avoiding as much math as possible by making the target number to hit equal to the value needed to hit.
But you probably already knew that. ;)