Why Gary Gygax Added Unrealistic Hit Points, Funny Dice, and Descending AC to D&D

In 1972, Dungeons & Dragons co-creator Dave Arneson introduced his Blackmoor campaign to co-creator Gary Gygax. The campaign stemmed from Gary’s Chainmail rules, but Dave’s game transformed the rules for miniature-figure battles into something new and irresistible—something that broadly resembled D&D.

My last post explained how Dave shaped a combat system that featured hit points, 2d6 to-hit rolls, damage rolls, and armor classes where higher numbers represented better protection.

Based on Dave’s demonstration, feedback, and notes, Gary added his own contributions to make the D&D game that reached print. In Pegasus issue 1, Dave recalled that Gary and his Lake Geneva group “had a lot more spare time than I did and they had a lot of ideas, so they came up with their own version of the rules.”

Gary changed Dave’s combat rules in 3 key ways:

  • Hit points became less realistic and more fun.
  • To-hit rolls switched to a twenty-sided dice, creating a new market for funny dice.
  • AC ratings flipped to make lower values better, forcing awkward, negative ACs on players.

Unrealistic hit points

Gary’s changes let characters gain hit points as they leveled. In Blackmoor, Dave wrote, “As the player progressed, he did not receive additional hit points, but rather he became harder to hit.” Dave based armor class on armor, but fighters gained better saving throws. By the Blackmoor rules, saves applied to weapon attacks, so fighters could avoid damaging blows. “Only Fighters gained advantages in these melee saving throws. Clerics and magicians progressed in their own areas, which might or might not modify their saving throws.”

In Chainmail, a hero fought as 4 ordinary soldiers and a superhero as 8. D&D translated this scheme by making heroes 4th-level fighting men and superheroes 8th level. When Gary reconciled Dave’s rules for hit dice with the notion of heroes that fought as several men, he probably decided to give characters more hit dice as they leveled. The mechanic seemed unrealistic. After all, nobody gets 10 or more times more durable through experience. But rising hit points helped power the game’s success. They boosted the positive reinforcement of leveling. Plus, heroes capable of unrealistically surviving many blows supported D&D’s combat-intensive, dungeon-bashing style. These advantages helped make the game so appealing.

Every “realistic” system to follow D&D echoed Dave Arneson’s original method of using hit points to measure a character’s body’s physical capacity to survive injury. In D&D, hit points rise as characters advance, and that turns hit points into an elegant damage-reduction mechanic. As characters level, they essentially reduce the damage they take from blows.

Using hit points for damage reduction boasts a number of virtues:

  • Combat plays fast because players do not have to calculate reduced damage for every single hit.
  • Although damage is effectively reduced, the reduction never makes a combatant impervious to damage.
  • Once characters gain enough points to survive a few blows, hit points provide a predictable way to see the course of battle. If a fight begins to go badly, the players can see their peril and bring more resources like spells and potions to the fight, or they can run. In a realistic fight, things can go bad in an instant, with a single misstep resulting in death.
  • Most attacks can hit and inflict damage, providing constant, positive feedback to players while everyone contributes to the fight. Realistic combatants do not wear down from dozens of damaging blows; instead each hit is likely to kill or maim. In more realistic systems like Runequest and GURPS, when two very skilled combatants face off, they block or dodge virtually all attacks. The duels turn static until someone muffs a defense roll and lets a killing blow slip through. This model may be realistic—it reminds me of those Olympic competitions where years of training turn on a single, split-second misstep—but the realistic model lacks fun. No popular sports begin as sudden-death competitions where the first to score wins.
  • Battles can gain a dramatic arc. Fights climax with bloodied and battle-worn combatants striving to put their remaining strength into a killing blow. No one likes to see the climactic battle fizzle with a handful of bad rolls, especially at their character’s expense.

Bottom line: Using hit points for damage reduction enables a combat system where you can hit a lot, and hitting is fun.

Funny dice

When Dave adapted the Chainmail rules for his Blackmoor campaign, he kept using ordinary 6-sided dice. He later explained, we had “no funny dice back then.”

The twenty-sided die may not have reached Dave’s corner of gaming yet, but Gary had funny dice and they enchanted him. At first, polyhedral dice only came from vendors in Japan and the United Kingdom, so getting a set required significant time and money. But by 1972, polyhedral dice started arriving from domestic sources. Gary recalled buying his first set from a teacher-supply catalog. In 1972, Creative Publications of California started selling 20-sided dice in a set of polyhedrals, and word spread among gamers. By 1973, Gary wrote an article touting funny dice. “The most useful are the 20-sided dice,” he explained. The original d20s came numbered from 0 to 9 twice, so most gamers rolled twice to generate a percentage from 1-100. Gary noted that gamers could do more. “Color in one set of numbers on the die, and you can throw for 5%—perfect for rules which call for random numbers from 1-20.” As an example, he mentions being “busy working up chance tables for a fantasy campaign game.” Gary found his new d20 so irresistible that he changed Dave’s 2d6 to-hit tables into D&D’s d20-based system.

Descending Armor Classes

As Gary reworked his attack table, he discovered that switching to descending AC numbers created a mathematical elegance. Game historian Jon Peterson describes how this system appears in a draft of the D&D rules. “If you were a first-level fighter rolling to hit, the number you needed was equivalent to 20 minus the armor class of your target. To hit AC 2, you needed an 18, to hit AC 3, a 17, and so on. Armor class descended to make it easy enough to calculate your needed roll that you wouldn’t even have to consult a table.”

If D&D had settled on this system, we might now be rolling a d20 to hit, adding the foe’s AC, and trying to reach a target number based on our character.

D&D reached players with a muddled system that kept descending armor classes, but hid any reason for the scheme. So players wondered why lower armor class represented better protection. Usually, bigger is better.

What happened?

When Gary expanded D&D to account for a greater range of levels than 9, he lost the mathematical simplicity. While the draft rules just present to-hit numbers for fighters up to level 9, the published D&D rules extend the table up to level 16 and beyond. To keep a steady advancement over a greater range of levels, Gary reworked the table and broke an elegant design. This left a system where players just used armor class to reference a row in a table and where intuitive, rising numbers could have worked just as well.

22 thoughts on “Why Gary Gygax Added Unrealistic Hit Points, Funny Dice, and Descending AC to D&D

  1. David Hartlage Post author

    I’ve never seen a sample of Dave’s to-hit table. He tended to keep specific rules to himself so he could make changes without involving his players. I suspect his to-hit table looked much like the Chainmail man-to-man combat table shown in my last post.

    1. OddJuniper

      Thanks for the heads up. The melee combat matrix ended up being the more interesting of the tables in that last post. I’ve largely played narrative/storygames the last several years, especially various PBTA games. Seeing Chainmail using a similar mechanic (2d6, with three outcomes) blew my mind a bit. Probably shouldn’t have.

  2. Jerry McDonnell

    Good one! I have d20 which is numbered 0 to 9 twice. I’m not sure where I picked it up, but I did so as a kid and I always thought it was some kind of gag to give to noobs, just to see how long it would take them to figure out why they are rolling so low.

  3. Michael Evans

    Nice aricle. As one of the Old Guard who experienced the birth of DnD from the beginning, it was fun to take a glance back toward the time where everything was new and experimental. I seem to remember 2 pre-Dragon magazines called Little People and Little Wars that were used to flesh out rules and settings. Any gaming source pre ’78 was always full of new concepts and ideas; ah, those were the times!

  4. Donald Ritchie

    THAC0 is easier than modern D20+ systems. With THAC0, you do all the math ahead of time. When you’re actually in a combat situation you never have to do more math in your head than + or – 10. If your THAC0 was 13, and you were trying to hit a dude in chainmail, you just had to subtract 5 from 13, or 8.

    On a D20+ system, the same fighter at the same level could have modifiers that total +13. Those modifiers tend to change more often than THAC0. You’re constantly adding a changing modifier against a different number every time you roll to hit and that combined number quickly exceeds 30 or more.

    Even my illiterate friend at 14 could subtract 5 from 13 quickly in his head. In practice, THAC0 combat rounds were also much more fast paced than D20 combat rounds.

    1. jodokai

      The simplicity of the system has nothing to do with THAC0, it simply had less modifiers. If you kept the modifiers the same, it’s easier to say my to hit is +13, I rolled a 10, so I hit AC 23. That is far more intuitive.

  5. mrswing

    I would have preferred the Arneson method, I think. I’ve found the unnatural hit points idea to turn combat into a slog from the very beginning (and I started on D&D and AD&D in the early ’80s. Combat is still more fun in theory than in practice… On the other hand, the idea was brilliant in that it was copied by every single computer game with level advancement ever.

  6. John Higgins

    >”If D&D had settled on this system, we might now be rolling a d20 to hit, adding the foe’s AC, and trying to reach a target number based on our character.”

    That is THAC0 described exactly. Some of TSR’s own rulebooks (namely late 90s introductory 2nd edition stuff) even explained how to use in that very manner.

    1. Erïch Jacoby-Hawkins

      Although THAC0 officially became a part of AD&D with the 2nd edition rule books, it was already being incorporated in some of the pre-2nd Edition modules in the mid to late 1980s, for example, modules I9 (Day of Al’Akbar) & I11 (Needle) from 1986 & 1987, respectively. I think it may have appeared in Dragon and Dungeon magazines around that time. The mechanic worked equally well in 1st as 2nd edition, as the AC system didn’t change and the principle of the to-hit tables remained the same.

  7. iain maclean

    We used to roll a d6 with our unmarked d20s (0-9) – a 1-3 on the d6 meant the roll stood as rolled, a 4-6 meant you added 10. Simple. And you got to roll two dice!

  8. Miguel

    Oh, the, the idea of THAC0 was there right since the beginning!
    ““If you were a first-level fighter rolling to hit, the number you needed was equivalent to 20 minus the armor class of your target. To hit AC 2, you needed an 18, to hit AC 3, a 17, and so on. Armor class descended to make it easy enough to calculate your needed roll that you wouldn’t even have to consult a table.”

    That’s essencially how THAC0 works 😉

  9. Ajax Plunkett

    THAC0 was referenced first in the Dungeon Masters Guide in 1979 by Gary Gygax on the monster stat sheets way in the back of the book. It predates 2nd edition and mid to late 1st edition modules. Gary knew what’s up. Sometimes….

    1. Erïch Jacoby-Hawkins

      Thank you for that reference. I knew I had encountered THACO years before 2nd-edition made it very explicit, but I recalled it from Dragon and some 1st-edition modules. But yes, I probably also did see it in the back of the DMG – I know I used that exact chart, because I copied all the monster XP values from it into their listings in my Monster Manual which had not included them. Probably it was something Gary used all the time and it was such a no-brainer to him that he forgot to mention it to the rest of us while he was composing his DMG. That would be just one of several unwritten or under-written conventions that were either common to most games, or limited perhaps to those in his immediate circle, things we have to remember nowadays to make explicit in newer edition guides.

  10. Griff

    I am curious about your sources for info on the dice. Arneson claims he found d20’s on his trip to europe in 1971. Gygax is well known for using poker chips for Tractics and also his early pre D&D games. Wesely claims he needed a D12 for his Strategos games earlier than both Arneson and Gygax. We’ve found games as early as the 50’s that had the the five shapes of dice, they existed as dice before D&D.

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  12. Numa Pompilius

    “In more realistic systems like Runequest and GURPS, when two very skilled combatants face off, they block or dodge virtually all attacks. […] This model may be realistic […] but the realistic model lacks fun.”
    So, Runequest and GURPS combat lacks fun? That’s a pretty bold claim. While I like D&D 3.5 and even Next more than GURPS, I find playing fighter in GURPS much more interesting than in any D&D edition.

  13. Ajax Plunckett

    Its interesting how game mechanics rely and what one has around them as building blocks. Technology , readily available items etc. the 6 sided die is used often in sub systems in early D&D. Later weapon damage was able to expand/ differentiate from 1d6 to allow variation of damage ( just to take as an example ).
    What do f the Dungeon Crawl Classics d3, d5 d7 etc were readily available in 1974 or 1977ish you probably not have weapons do 1d4+1 or 2nd ed. Rappers doing 1d6+1 but rather 1d7.

    What would RPG’s use in a what if 1950’s beginning? A deck of cards, tarot cards with d6’s.?

    And then of course a sort of ” roll back ” of too specific random numbers as percentage dice for thieves skills turns to d6’s in some OSR games and of course D20 for a lot of mechanics in post 2000 3rd ed D&D.


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