Saturday 2 June 2012

DnD Next Armor Class

Plate Armor with FABULOUS neck-protection
In my last post, I promised a more in-depth look at DnD Next's AC system. In order to do that, let's go back a bit, to the days
When men were men, and men used charts!

Attack Matrices

In the first editions of DnD, in order to determine if an attack hits, each class came with an attack matrix showing, for each level/AC, what number you needed to roll to score a hit. So the process is:
  1. Calculate To Hit Value=(chart lookup)
  2. Roll=d20 + bonuses  
  3. Compare values


The idea of a chart that each player needed to have available at all times got old pretty fast, such that the DMG in 1e already mentions calculating a character's THAC0 or "To Hit Armor Class 0".  So here the process is:
  1. Calculate To Hit Value=THAC0-AC
  2. Roll=d20+bonuses(level,proficiency,environment,effects)
  3. Compare values

Now this is a subjective improvement, some players prefer charts, but my impression is that THAC0 was eventually widely adopted and that most players, including myself, prefer it.

DnD Next

DnD Next changed the AC system so that a character's AC IS the To Hit value. As such the process to determine a hit is:
  1. Roll=d20+bonuses(level,proficiency,environment,effects)
  2. Compare to AC
I would argue that this is an improvement on the THAC0 system for two reasons:

First, in the THAC0 system, I need to calculate and remember two values for each attack, which for my notoriously bad short-term memory, is a bit clunky. I didn't realize how much the 5e version is an improvement until the 5e playtest. It's nice not having to calculate the to-hit value for each specific monster.

The second reason this is an improvement is that the DM need not reveal the monster's AC to the players.  The player just states his roll and the DM compares to the AC value.  This is more immersive, since the player doesn't know objectively how hard the monster is to hit.  Monsters should be mysterious and an unknown quantity.  Once they become just a list of numbers in the players' mind, then the game becomes less immersive.

Actually, this AC as the to-hit value may have already been present in 4e, but 4e combat is so Byzantine and complex that I didn't notice how great it is until now!

Anyway, I still won't be giving up Labyrinth Lord as my choice version of DnD to run, but I might just swipe 5e's Armor Class rules when I do run my next DnD game.


  1. Ascending AC started with Third Edition.

    It does save a step, but with ascending AC comes the sense that AC can increase without limit, and as such is part of the general trend towards target number (and bonus) inflation that was so prevalent in 3E (and in 4E, though to a slightly lesser degree).

    Also, originally AC was not a target number, but a literal class that armor belonged to. You could map the number back to the armor in question, which allowed for things like weapon versus AC tables.

    For example, the only way to get an AC of 5 was from chain. AC 4 was always chain + shield. AC 3 was plate. AC 2 was plate + shield. Etc. Dexterity did not originally affect AC.

    The difference is subtle but real.

    1. Oh, so that's Ascending AC. I'd heard it mentioned before but admittedly never seen it in play till now. I missed 3E and am currently playing in a 4E game, but I just used a character generator to generate the stats(couldn't be bothered to read an entire book on my class!).

      So for my next post I was planning on writing alternative AC rules for Labyrinth Lord based on this. Instead, I think I'll just write an extension to Descending AC rules which uses this principle.

    2. Both Lamentations of the Flame Princess and Swords & Wizardry (optionally) use ascending AC too, so you might want to check them out. Links to various free PDFs:

      (WhiteBox is an OD&D emulator.)

    3. Thanks for the links. Yeah, looks like S&W does exactly what I want with it's two compatible options.