Armor Class (AC) is an abstract rule system used to determine one's ability to get hit in combat. The method of which varies from edition-to-edition. This has caused minor problems in the past, due to DMs picking-up an adventure module that is not of the right rule edition, but this was mostly a learning experience in remembering to applying rule conversions.
Original Dungeons & Dragons
The original 1974 Dungeons & Dragons rules referred players to the use of the Chainmail medieval-fantasy wargame for movement and combat. Chainmail had three ways of handing combat: Mass-combat; "man-to-man" fights; and "heroic" monster slaying. While the third one would make sense for D&D and allow for characters to take-down powerful characters and monsters with a single roll, it was an all-or-nothing engender nor did it allow for troops, weaker monsters (Goblins, Orcs, Skeletons, etc.) or low-level (1st-to-3rd) characters to participate. Instead, The second system was embraced.
The "Man-to-Man" system used Weapon Class (WC) and Armor Class. Weapon Class was based not on the weapon's power or armor penetration, but on its reach. Having a higher Weapon Class that ones opponent grants first-strike. Armor Class was based on bread classifications of armor types, with the medium-grade armor called "Chain, Banded, Studded, or Splint Armor" Under the "Individual Fires with Missiles" table, the Armor Classes are noted between 1 (no armor) and 8 (Plate Armor & Shield), plus Horse with "No Armor" and "Barded".
The original Dungeons & Dragons rules also introduced an "alternative" to-hit system based on Character class/level vs opponent's Armor Class. Unlike the Chainmail system, base AC was 9, going down to AC2 (Plate Armor & Shield) -- this "descending AC" approach, with lower AC being better, would become standard for D&D until the 3rd edition. Armor Class in this edition was not meant to be adjusted high/low Dexterity nor magical enchantments, so that a character ether used the AC of a shield spell or his or her own armor, whatever is better. Magical armor did not improve the wearer's AC, but instead it lowered the opponent's Hit Dice, thus making it less likely to hit. The point is, AC remains static.
Some monsters have AC so high (or low in number), that it goes past 0 and into the negative numbers. eg: 2, 1, 0, -1, -2, etc.
|Chainmail Armor Class||OD&D Armor Class|
|No Armor||1||No Armor or Shield||9|
|Leather or Padded Armor||2||Shield Only||8|
|Shield only||3||Leather Armor||7|
|Leather Armor + Shield||4||Leather & Shield||6|
|Chain, Banded, Studded, or Splint Armor||5||Chain Mail||5|
|Chain Mail + Shield||6||Chain Mail & Shield||4|
|Plate Armor||7||Plate Armor||3|
|Plate Armor + Shield||8||Plate Armor & Shield||2|
Basic Dungeons & Dragons
Just like the rest of the rules, AC in the "Basic" line of D&D products were simplified and consolidated. Base AC was still 9 and "descending". This time AC could be adjusted by high/low Dexterity and magical enchantments. Armor Classes is as OD&D Armor Class, above.
Advanced Dungeons & Dragons
Much like the rest of the rules, Armor Class in the "Advanced" line of D&D is altered and greatly expanded upon. Base AC is now at 10 and more armor is added to fill in the "[armor] & Shield" gaps. AC remains 'descending' with the best Armor Class in this edition maxed at -10. Both first and second edition AD&D used the same AC system. While first edition used a series of class-dependent 'attack matrix' hit tables, second edition introduced THAC0 as a means to standardize a character's or monster's chance of hitting a target with an Armor Class of 0.
1st and 2nd Edition AD&D Armor Class Table
|Leather armor or padded armor||8|
|Leather + shield, padded armor + shield, studded leather armor, or ring mail||7|
|Studded leather + shield, ring mail + shield, or scale mail||6|
|Scale mail + shield, chain mail, or elfin chain mail||5|
|Chain mail + shield, elfin chain mail + shield, splint mail, banded mail, or bronze plate mail||4|
|Splint mail + shield, banded mail + shield, bronze plate mail + shield, or plate mail||3|
|Plate mail + shield or field plate armor||2|
|Field plate armor + shield or full plate armor||1|
|Full plate armor + shield||0|
Third Edition Dungeons & Dragons
As like the rest of the rules for the Third edition of D&D, AC had a major overhaul. Base AC is still 10, but now it is "ascending", with higher AC being better and armor, shields, high Dexterity, and magical enhancements now adding to the base AC score. This method will continue on to the Fourth and Fifth Editions.
|Light Armor||Medium Armor||Heavy Armor||Shields|
|Leather||+2||Scale mail||+4||Banded mail||+6||Shield, small||+1|
|Studded leather||+3||Chain mail||+5||Half-plate||+7||Shield, large||+2|
|Chain shirt||+4||Breastplate||+5||Full plate||+8||Shield, tower||(cover)|