AC 10

Armor Class in D&D
The transition from original "white box" D&D to Advanced took place over approximately two and a half to three years - the first Advanced material is published 1977 and the Game Masters Guide seeing print in late 1979. Until the Game Masters Guide is available AD&D remains incomplete because the combat charts needed for play are included in that volume. During the transition period, those of us playing the game were confronted with a dilemma - how to go about combat without the new charts. This problem is exacerbated by the existence of AC 10 in the Advanced system.
Armor Class or AC as it was frequently abbreviated has been a part of D&D since the beginning. The alternative combat system (d20) included in the White Box Little Brown Books (page 19, Vol. 1) includes Armor Class 2-9, with AC 9 representing "No Armor or Shield". The Advanced Game includes an Armor Class of 10 defined as "None" - meaning no armor or shield. AC 9 is redefined as "Shield only" (page 39, PHB). To make matters more confusing, Basic/Expert D&D is similar to ODD/White Box in the use of AC 9 as "Clothing only" (page B12 of Moldvay Basic).
A comparison of AC across early editions reveals that chainmail is AC 5 in the Original Game, Advanced and Basic/Expert. Plate(mail) Armor is AC 3 in each of the three versions, but Leather Armor is AC 7 in the Original Game, and Basic/Expert, but is listed as AC 8 in the Advanced PHB. The Advanced Game includes a number of types of armor not included in the Original Game or Basic/Expert such as studded leather, ring mail, splint or banded mail, and padded armor. A shield gives minus one to AC across these editions.
The result is that it is the less armored AC values including ACs 6, 7, 8, 9, and 10 where the difference in Armor Class values will occur. I can only assume that part of the reason for this disparity lay in the desire for greater granularity in the Advanced Game. Part may also have been in order to distinguish AD&D as a separate game from the D&D line. The result is that using modules and play aids written for AD&D can require a bit of AC adjustment on the part of the referee and that until the Game Masters Guide combat tables are released in 1979, we had to extrapolate (add one) to arrive at the numbers needed to hit AC 10 using tables that only went to AC 9.
As a determining factor (along with class and level) in the combat odds reflected in the number (or higher) that needs to be rolled on a d20 in order to score a damaging "hit", AC is a central factor in the game. Monsters lacking "armor" as such still have an Armor Class, which often reflects factors other than "armor" and which may affect their ability to avoid damage, factors such as size, toughness, immunity to pain, or agility which can improve their ability to avoid damage (or its effects) therefore making them harder to score an effective "hit" against during combat.
The d20 combat mechanic in D&D is an abstract "game" representation of armed conflict. There is no real relationship between game mechanics and actual life and death sword-play regardless of the RPG system, but the designers of D&D across all editions strive to produce an exciting and believable game representation of such. Using miniatures or theater of the mind, the popularity of the game seems a good indicator that the designers have achieved their goal.
Armor Class (or AC) is a constant (and defining) feature of the D&D line of games, and until the 3rd Edition lower AC is better AC. The 2nd Edition of the Advanced Game introduces THAC0, which stands for To Hit Armor Class Zero, as a streamlined method to calculate the combat number needed to be rolled on a d20 in order to effectively cause damage to an opponent. The 3rd Edition (dropping the "Advanced" name) introduces ascending AC which becomes in effect a target number for the d20 combat roll. Use of ascending AC continues in 4th and 5th Editions and while perhaps more intuitive, I feel a personal kinship with the old descending armor class system - under which I have experienced many memorable tabletop adventures.