THE D&D IN MY HEAD: In Only 6 Load-Bearing Numbers

art: Mark Tedin

One of the most frustrating questions for me to read is, "Is X-OSR system compatible with Y-OSR adventures?" or "Can I use BX rules with AD&D modules?" or "Can I use OSE with Dolmenwood?" (Good lord! Y'all! Dolmenwood = OSE = BX)

I find these questions frustrating because while there are differences in all those systems they are very minor. And those differences will influence gameplay by 0 versus the "chaotic" nature of a d20 roll at crucial points in gameplay ✤ . 

Meaning, it doesn't matter if your save vs Death is supposed to be a 13 or 14 given you'll have to roll a d20 against it. Fight me math nerds, but better to be off by 1 or 2 points in some score than never get the game to the table out of fear of "doing it wrong". 

This agonizing also tends to sorta soft-lock away what is otherwise exciting and amazing content (Lair of the Lamb looks cool but I don't play GLOG, I play BX) and reinforces the idea that the math in D&D (and most other RPGs) needs to be so finely balanced for fairness or to make sure the game is not broken (Sigh...I want to run the dungeon but I need to write down stats for every monster and what how do I do traps again? And what if it asks for a save, my system doesnt use those)

Being confident enough to run things "just good enough" to get to the playing of "D&D" is an important skill of any DM. Once you hit that point it is very freeing because your brain power is not chewed up agonizing over system rules and details and can be put to more creative things.

And the next time someone says "Oh I've always wanted to play D&D but never got the chance", you can cannibalize their board games for dice and minis, download a Dyson Logos map, and start playing right there at their table.

To play roughly 0D&D, "Holmes Basic", BX, BECMI, 1eAD&D, & 2eAD&D and by extension the retro-clones White Box, Delving Deeper, Sword & Wizardry, LotFP, Basic Fantasy, and my fav OSE or to run adventures made for one of these with the rule systems of the other, you really only need to know 6 things:

✦ Marcia over at Traverse Fantasy also has outlined a similar "unified language" for D&D with clearer language than my back-of-the-envelope-scrawl; she also discusses unified procedures which I avoid here for breavity ✦ 

Roll for stats using 3d6 down the line and determine where you get the +/-1 bonus: 

  • 3d6 down-the-line (Or roll a d4, d6, d8- its still the same 3 to 18 range)
  • Easy to remember is this pattern:
    • 18 (+2)
    • 17-13 (+1)
    • 12 to 09 (+0)
    • 08-04 (-1)
    • 03 (-2)

1 HD Monster = Level 1 Fighter

  • So a 4 HD monster is the same as a level 4 fighter; 0 level people are 1d6 per HD
  • A Monster HD is d8 and monster damage is d6 and a Fighter HD is generally a d8
  • If you ever need anything for a monster, look at the fighter class for suggestions
  • Whatever math you give a monster, you give a fighter the same; so if a 3HD gnoll hits with a +2 then a 3rd level fighter should also get a +2 to-hit

Classes are basically: Use all weapons/armor no magic or Use magic no armor/small weapons:

  • Fighting types use all weapons and armor and have the biggest HD; advance on with (X)xp
    • Cleave: If you kill an opponent, move 1 square and make another attack; repeat equal to your level; I really started to feel this helps give fighting types a little
  • Magic-types use only small weapons, no armor, and have the smallest HD, but cast spells; advance with (X*1.25)xp
  • If you combine both broad abilities (sans Cleave) into one class, a la the elf, advance with (X*2)xp
  • X is experience points, coins, monster heads, or "completed adventures", seasons ect

Weapons and Armor: Damage 1d4-1d6-1d8; AC 10, 12, 14, 16

  • Small weapons (dagger) 1d4, Average weapons (sword) 1d6, Large weapons (battle axe) 1d8
  • Or simply, weapons 1d6 dmg: sword (melee), spear (reach), bow (range)
  • Roll a d20 vs the following target armor numbers: None is AC 10, Leather/Gambeson is AC 12, Chain is AC 14, and Plate is AC 16; shields are AC +1

Saves: 14 is the number of fate and the gods ✤✤

  • The middle number for saves in D&D is 14
  • "Death" saves are 12 (because you want to make it easier to avoid a one-shot kill) and most "Spell" saves are 16 (because you want most spells to take effect)
  • So basically: Death (12), Trap (14), Spell (16); Hardy or lucky backgrounds like "Dwarf" or "Halfing" or whatever your fox-based original class is lower their saves by -4.
When in doubt, its a 1-2 on a d6 (or 30%) that thing will happen
  • Open a stuck door
  • Check if the PCs/Monsters are surprised
  • Some random chain reaction happens
  • Want to compress all the thief skills into a single roll (mod by DEX)
  • Want to know if a character's background helps them (mod by WIS)

✤ And this is not a flaw in game design math nerds. Its fun, chaotic, and creates memorable moments at the table. The "Nat 20!" meme sticks because of that feeling.
✤✤ Ava from Permanent Crainal Damage has smartly suggested that perhaps the three saves are best defined as Monster (12), Trap (14), Spell (16) which align with the 3 broad classes of threat in D&D-- smart! This also aligns with the random stocking procedures of BX: 1-2 Monster, 3 Trap, 4 Special, 5-6 Empty.


  1. > better to be off by 1 or 2 points in some score than never get the game to the table

    *Hard* agree on this.

    I've run a lot of Into The Odd ect. and have come up with effective conversion systems ─ my advice to others is "come up with a system: it's better to be workable than "correct"."

    Hell, correct is a false god anyway

    1. That is a good point. Whatever you choose to be consistent for *play* is the correct choice.

  2. I am DMing a campaign with OSE and DCC characters in the same party, through an original Basic module. No problems whatsoever.