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Message: <blockquote data-quote="rogueattorney" data-source="post: 5800468" data-attributes="member: 17551">Some historical observations:Armor Class (in OD&D without the supplements) was originally just that.  It was the class of armor you were wearing.  It was a static, discreet number that didn't vary unless you changed what armor you were wearing.  There were alterations to your chance to hit depending on circumstances, but there were no alterations to the AC.  Thus, the AC number was just a position on a chart and could have been labeled anything, A through H, 1 through 8, whatever.  In Chainmail, the armor types were labeled 1 through 8 on one of the charts, but no real reference to AC as a number was made and those numbers didn't have any relevance to the game.  You'd look up the type of unit that was attacking on a chart, cross reference its target and roll a 2d6.  If you hit the target number or more, you hit.  In the one-on-one rules for the fantasy supplement, you'd get weapons' chances to hit certain armor types and monsters.  These "to-hits" weren't strictly linear.  For example a weapon may do better against chain than plate or a shield may be more effective against certain weapons than others.  (The weapons v. AC table introduced in the Greyhawk Supp. was an attempt to re-incorporate the old Chainmail charts back into D&D.)When OD&D came out, it pretty much worked the same way, except that an "optional" combat system was included in which you rolled a d20 to hit a target number and these target numbers were now linear.  The armor types were given "Class" numbers of 2 through 9 (from best to worst).  But those numbers didn't really mean anything mechanically, and AC didn't change.  Magical armor was a penalty to your opponent's chances to hit you.  Dexterity didn't alter your chances to be hit.  So, again, AC was just a label on a chart.A lot of people have tried to track down where, exactly, the AC numbers came from, and it seems they originated with a navel combat game that Dave Arneson had played.  I haven't heard anyone explain how those numbers were used in the naval combat game, but a few people have made the reasonable guess that to hit you were supposed to roll the number or lower on a 2d6.  That would explain why armor got better as the numbers got lower and why the numbering ended at 2.It was with the Greyhawk Supplement that AC became something that could vary with some other factor than what armor you were wearing.  Dexterity modified your AC, as did magical armor.  So, for the first time, "AC" did not directly relate to what type of armor you had on.  AC5 could be Chain Mail, or it could be someone with Leather Armor and Shield +1.  This meant the hit chart had to include numbers less than 2.AD&D further complicated things by having different types of armor having the same AC (leather + shield and studded had the same AC, for example), which made the Weapon v. Armor Type adjustment table make a lot less sense.  It also added another column to the chart (no armor went from 9 to 10, leather from 7 to 8, but chain stayed the same at 5).  Further, as AC improved into the negative numbers, the "to hit" chart diverged from its strictly linear composition.  This was to give pcs a shot at hitting certain extra-planar entities and monsters a chance at hitting super well-equipped players.So, by the time AD&D came along, you had a larger, more complicated to hit chart.  To help DMs out, the mnemonic, THAC0 was used, so that the DM didn't have to always go back to the more complicated charts.  It really only worked when dealing with ACs above 0, and the DM had to remember that when ACs got into the negatives, they needed to go back to the charts to deal with the repeating 20s.  In other words, THAC0 wasn't the rule, it was just a tool to be used at the DMs convenience (or not).  AC was still just a label on a chart and could have been relabeled whatever.With 2e, THAC0 became the rule.  To hits became linear and it always worked, regardless of whether the AC was negative or positive.  For the first time, AC became a variable in an equation - there was mathematical value to AC.  Also, 2e was the first time in my experience that negative ACs became commonplace in the game.  And even then - again, in MY experience - it wasn't at the game table, but rather it was in the Baldur's Gate style video games that everyone seemed to have a negative armor class.When everyone's AC was between 2 and 10, THAC0 wasn't all that hard to work with.  When ACs dipped into the negatives, with no real floor.  That's when it got a bit silly.Thus, my point...  THAC0 was an artifact of game design that had been abandoned fairly early in D&D's development.  The 2 to 9 (or 10) scale probably should have been abandoned with the Greyhawk Supplement or with the 1e PHB.  Speaking as a confirmed grognard, the loss of THAC0 or "backwards" AC isn't really a big loss.The real big, important change with 3e - which continued with 4e - that gets lost in THAC0 and backwards AC discussion was the lack of any sort of set range of AC.  However you wanted to label it, prior to 3e, AC had a range of about 15 slots.  In 3e terms, AC in older editions never got much better than 25 and even that was rare; in practical terms pcs were going to level off at about 22.</blockquote>

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