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From a game design standpoint, are there any advantages to descending AC? I've tagged this D&D because I think this was the most popular game which required a low AC all the way to the year 2000 when 3e went with ascending DC.

I don't know if there are any first hand sources from Gygax or Arneson explaining why they went with it (and by extension, THAC0), but I wonder if there are general reasons for it.

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  • \$\begingroup\$ I've shot off that answer and people are upvoting it, but I'm not sure I've hit your "from a game design standpoint" button. If I haven't, please can you say a bit more about what you mean by that and I'll try to alter my answer accordingly. \$\endgroup\$ – harlandski Feb 1 '15 at 15:19
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    \$\begingroup\$ You might be interested in a unified notation for ascending and descending AC. \$\endgroup\$ – Lexible Feb 1 '15 at 16:09
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    \$\begingroup\$ @harlandski It actually does hit that point, by showing how there is no design standpoint to it. Sometimes the best answers are the ones that clear up a misapprehension in the question. \$\endgroup\$ – SevenSidedDie Feb 1 '15 at 19:14
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I'm going to answer this purely from a D&D perspective, as this is my area of expertise.

The idea of 'descending armor class' in D&D comes from the 'alternative combat system' presented on p 19 of the first D&D book Men & Magic (1974). Here the armor classes given range from 9 (no armor or shield) to 2 (plate armor and shield), with the other column having the to-hit number on a twenty-sided dice for fighting-men (sic) of groups of levels 1-3, 4-6 up to 16+.

The armor categories in the 'alternative combat system' closely mirror those of the Man to Man Melee table on p 41 of Chainmail, the game which in theory was needed to play the first edition of D&D, but in fact could be sidestepped with the 'alternative combat system'. The Chainmail table differs however in that no numbers are given to the armor classes, only descriptions (e.g. "No armor", "Leather or padded armor"), and compares this to the weapon being used to attack, there being no concept of class or level in Chainmail. The 'Individual Fires with Missiles' table on the same page of Chainmail actually refers to these as 'class of armor', but with an ascending numbering system, from 1 (corresponding to no armor) to 8 (corresponding to plate armor and shield).*

In the OD&D 'alternative combat system', to hit progression against certain armor classes was not completely linear as you advanced in level, nor would it be so until the idea of THAC0 was made part of the official rules in the AD&D 2e Player's Handbook (1989) and Dungeon Master's Guide. (THAC0 is mentioned in an appendix to the AD&D 1e DMG, but still the tables for hits against certain armor classes are not completely linear in AD&D 1e). From OD&D to AD&D 1e the rules assumed you would consult a table to calculate your hit probability.

By the time of AD&D 2e the idea of to-hit against armor class had become nothing more than a calculation. Your THAC0 decreases in even steps depending on your class and level. Then: THAC0 - AC = to hit number. See for example p 89 of AD&D 2e PHB. From here it was a short step to the much simpler system of an ascending armor class equalling the to hit roll, with modifiers added to or subtracted from this roll, which was first introduced in 3e (2000 ff).

So there was no particular advantage to the 'descending armor class' system. It had just developed historically from its roots in Chainmail from being a 'this weapon against that armor' to a 'this level of ability vs that armor' calculation. Mathematically there is no difference between the principles of AD&D 2e (descending) and D&D 3e (ascending) armor classes, once a graded THAC0 increase by class and level was introduced and players no longer had to refer to tables to calculate the possibility of a hit.

* Thank you to Jon Peterson for highlighting this last connection. For a much more detailed discussion of the historical development of armor class see his book, Playing at the World (2012), and for his treatment of armor class in the various editions of Chainmail, see: http://playingattheworld.blogspot.com/2012/10/armor-class-in-chainmail.html

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  • \$\begingroup\$ Don't Give Up the Ship (1972) - BoardGameGeek. The credits list Arneson first, Gygax last, which probably means Arneson was primary or sole author. \$\endgroup\$ – SevenSidedDie Feb 2 '15 at 7:20
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    \$\begingroup\$ Jon Peterson, the author of Playing at the World (2012), passed on some useful information in personal correspondence with me. I quote, "Don't Give Up the Ship as published in 1972 was written by Arneson, Gygax and Mike Carr. Arneson and Gygax agreed to start working on it together in 1969. Mike Carr helped patch it together and put in some advanced rules in the last year or so of development. Anyway, that game does not contain armor class." \$\endgroup\$ – harlandski Feb 3 '15 at 14:57
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There's one useful advantage to descending AC, but it clearly wasn't employed for the versions of D&D that did use it.

Your attack roll can be simplified to D20 + Attack Bonus + Target's Armor Class vs. 20 (or 21)

Having a static target number to meet isn't a bad choice at all, design-wise. That said, the older versions of D&D that used descending AC didn't also have positive attack bonus advancement, but rather, kept ratcheting down the target number (THACO).

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    \$\begingroup\$ I have a house rule because I feel like shields alwasy got shafted in D&D, which is that a shield halves your armor's AC in descending AC rules. Unarmored at AC 10? Shield brings you to 5. Plate mail at AC 2? Shield puts you at 1. Shields benefit unarmored people way more than armored, but it makes them generally more powerful. This is only easy to do with descending AC. \$\endgroup\$ – user9935 Feb 2 '15 at 7:08
  • \$\begingroup\$ I like this answer as, once I realized it, the whole AC system with its weird and arbitrary numbers (0 armor = 10 AC? and THAC0? Why not THAC10?) became much easier to comprehend. AC is just a to-hit modifier. Suddenly you become your group's "what do I roll to hit" guy. \$\endgroup\$ – Smithers Feb 2 '15 at 18:23
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    \$\begingroup\$ Yeah, I saw one of the early OSR clone games use this and everything fell into place. I was actually kind of angry that it didn't show up earlier, as THACO and hit charts was one of the unnecessary clunky bits to D&D. \$\endgroup\$ – user9935 Feb 2 '15 at 23:31
  • \$\begingroup\$ That's actually kinda neat, especially because Shields always feel a bit weird (not that +2 AC in 5e is bad, it just seems tacked on) \$\endgroup\$ – Michael Stum Feb 3 '15 at 3:43
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I'm glad that my historical answer above is popular, but I have another take on things which I will offer, purely from a 'game design' perspective. As it is much more subjective than my other answer, I would rather separate it off from my other answer. Again I'm limiting myself to how "negative armor class" functioned in the pre-3e iterations of D&D.

One advantage which "descending armor class" offers is that it separates the character's class and level ability from other factors, which possibly makes those other factors seem more meaningful to players.

If we take the simple example of a level 10 fighter trying to hit someone in full plate, with no negative modifiers. In AD&D 2e, the fighter has a THAC0 of 11. Full plate has an AC of 1, so that brings the to-hit number to 10. Then the fighter adds any bonuses, including bonuses for high strength, situational modifiers, spells etc to his roll. This will not be a very big number, almost certainly below the 9 needed to guarantee a hit. (Critical fumbles are an optional rule in 2e, see DMG p. 61.)

In 3e, full plate has an armor bonus of +8, meaning a person wearing full plate has an AC of 18. The 10th level fighter has a +10 base attack bonus. Then he adds his (most likely more generous) strength bonus to attacks, and other bonuses for situation, spells etc, obviously resulting in a number above 10 and quite possibly approaching the 16 needed to guarantee a hit provided a natural 1 is not rolled. This large number is added to the roll.

Apart from the increased power levels in 3e, there is nothing mathematically different happening here. Yet there is perhaps a difference in feel when the bonus you add to the dice roll is not such a big number, as it does not include the benefit given by level and class.

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