Armour Class is one of the more unusual of Dungeon & Dragons mechanics. As far as I am aware, it is a system first used in naval miniatures games, particularly by Fletcher Pratt and Jane’s Fighting Ships. Dave Arneson designed a naval game himself which used it, and from there it passed into Dungeons & Dragons. You’ll find no actual mention of the numeric Armour Class system in Gygax’s and Perrin’s Chainmail; instead, it just uses a table that cross references the type of armour with the weapon used.

The Armour Types of Chainmail are as follows: Armours: None, Leather/Padded, Shield, Leather + Shield, Chain/Banded/Studded/Splint, Chain+Shield, Plate, Plate + Shield

With the publication of the original D&D set, these armour types were given armour “classes” of between 2 and 9. These are particularly odd values to give the types – where is AC 1, for instance? (I presume that the Full Plate & Shield had that value, but was omitted from the published text).

It is worth noting that Dexterity gives *no* modifier to AC, a magic suit of armour subtracts its value from the “hit dice of the opponent”, and magical shields have only a 33% chance of working (in which case they give their penalty to the attacker).

Supplement I: Greyhawk greatly increased the number of magical armours in the game, but Gygax was still somewhat struggling with how Armour Class would work. As a result, the explanation for how magical armour and magical shields work together is extremely clumsy. Gygax used this table to illustrate – which is not particularly illuminating!

(Not surprisingly, it also has errata: Chainmail and +1 shield should be AC 3).

Gygax was a lot clearer in Advanced Dungeons & Dragons. Although the text is still rather confused about whether magical bonuses and penalties apply to the die roll or the armour class, the tables range from an Armour Class of -10 to +10, and this would be the main range of Armour Class values for the next twenty years or thereabouts.

One of the interesting features of this is that every AC in original D&D (sans supplements) is in the range of 2-8. Greyhawk‘s expansion of the AC ranges allowed the Will-o’-the-Wisp to have an AC of -8, the highest on the supplement’s table, but there aren’t many other monsters with negative ACs (the Platinum Dragon, with -3, is the next best).

Blackmoor is back in the 2-8 range, Eldritch Wizardry gives the first AC of actually 9 (the Succubus), and gives Demogorgon a fearsome -8 AC, with Orcus behind on -6. Finally, Gods, Demigods and Heroes mostly goes back to the 2-9 range, with a few exceptions (and not gods – generally monsters), though some of the gods have magic armour that isn’t factored into their AC – Odin has a helm +5 and mail +5! (This work, primarily Ward and Kuntz, may be based more on original D&D than the supplements!)

AD&D is an interesting beast. It has many, many problems which I tend to pick on, mainly due to a lack of good development and editing work – in fact, the development is most haphazard. Gygax was distilling all the work done on D&D into one coherent system, and to a large part he was successful. To my mind, his biggest problems come when he invents new material (such as the initiative system) rather than adapting the old. Interestingly, there is no AC 10 in the Monster Manual (but, to be fair, there’s no AC 9 in the monsters in core OD&D either!) The Will-o’-the-Wisp retains its AC of -8, likewise the Platinum retains -3. Demogorgon and Orcus keep their respective ACs.

The tables in the DMG enshrined the -10 to 10 range of ACs, and a monster came out with a -10 AC not all that long after – Dave Sutherland’s take on “Lolth” – from Q1. Also in 1980, RJK’s and Jim Ward’s revision of the deity supplement gave the gods ACs that weren’t generally quite as impossible – Odin has AC -6 instead of his previous (effective) -8, although we get our first breaking of the -10 “cap” – Indra has an AC of -12, which is correctly calculated as he possesses a suit of +4 plate, a +4 shield, and has a 25 Dexterity (-6 defensive bonus).

However, it is debatable how many of these really low ACs belong to beings that will be fought. The majority of D&D and AD&D play will be against opponents with good ACs of 2 or thereabouts.

One of the very interesting features of having almost every (fightable) monster in oD&D/AD&D being in the 2-9 range of AC is that it makes all characters relevant in physical combat, something that quickly became not the case when you dropped into the negatives.

Speaking in terms of THAC0, first level magic-users and thieves in AD&D had a THAC0 of 21; this translates into a THAC2 of 19 and a THAC5 of 16. For the thief, this doesn’t change until the character reaches 5th level, at which point it improves by a massive 2 points (THAC2=17, THAC5=14). At 9th level, the thief’s next point of improvement, this improves by 3 (THAC2=14, THAC5=11).

Of course, using level is quite an inaccurate guide in AD&D, as all characters advance at different rates. Here’s a look at the THAC0s of characters dependent on Experience Points – a more interesting guide.

Here’s a list of the THAC0s at key XP points:

As you can see, the disparity between the cleric and the thief isn’t actually that bad – and even the fighter rarely opens up much of a lead. In Second Edition, the 19->16 jump isn’t there for the thieves and MUs, but they start at THAC0 of 20 instead. The thief also has the possibility of the backstab – a +4 to hit – which brings their effectiveness close to that of the fighter.

The fighter’s curve is very slightly ahead of that of 4E – getting a +2 weapon at ninth is by no means assured, and the only other means of increasing your attack bonus would be the rare Gauntlets of Ogre Power or Girdles of Giant Strength. (Of course, it is way, way behind that of 3E, which is one of the reasons that Pathfinder is – for me – a lot of great material built on foundations of sand).

(The biggest problem with the thief in AD&D simply comes from their ludicrously low chances of success with their thief skills at low levels. With only a 20% chance of finding and removing traps at first level, why are they with the group? At higher levels, magic begins to tread on their toes.)

Where the AD&D system begins to break down is at the very high levels (11+), but it serves pretty well before then in my opinion. Negative Armour Classes do prove somewhat of a problem, which I’ll discuss in a later post.