THAC0 in B/X Essentials

Over the last weeks, since the publication of B/X Essentials: Monsters, I've seen some online grumbling about my inclusion of THAC0 in the monster stat blocks. Some people have even gone so far as to say that this makes B/X Essentials incompatible with B/X, thus utterly defeating one of the aims of the project. So I wanted to address this.

To begin with, it's worth restating the aims of the B/X Essentials project:
  1. 100% accurate rules clone.
  2. Organised for quick reference.
  3. Clarified rules.
  4. 100% Open Game Content.
The question of THAC0 lies somewhere between points 1 and 2.

Next up, let's look at an example monster from the B/X Essentials monsters book alongside a monster from the original books.

Doppleganger in B/X
Doppelgänger in B/X Essentials

Note the differences in the B/X Essentials version:
  • The average hit points for the HD are listed.
  • The THAC0 is listed.
  • The full set of save values are listed, not merely the class and level that the monster saves as.
  • The pre-calculated XP value is listed.
So the stat blocks in B/X Essentials: Monsters are definitely different to traditional B/X stat blocks. They contain extra information. Here's where the second project aim stated above -- “Organised for quick reference” -- comes in. The core B/X Essentials books form a 100% accurate rules reference for B/X, but the way the rules are presented is deliberately different. My aim is to improve the presentation of the rules, making them easier to understand and easier to reference.

Including extra information in the monster stat blocks was done for exactly this reason: to make the monster entries easier to use as a reference during play. Let's look at each of the extra elements:

Average hp: Provided for convenience, either for quick usage instead of rolling for a monster's hit points, or to quickly see whether a rolled hp total is above or below average. The average values are calculated directly from the monster's HD.

THAC0: Provided as an option for referees who prefer to not use the monster attack table from B/X Essentials: Core Rules, thus reducing the need to look up info about a monster across multiple pages. The listed THAC0 values are lifted directly from the monster attack table.

Full saves: Provided in addition to the traditional “save as” value, as an option for referees who prefer to not look up monsters’ saves in the class saving throw tables, thus reducing the need to look up info about a monster across multiple pages. The listed save values are lifted directly from the appropriate class’ saving throw table.

Pre-calculated XP: Provided for convenience to save referees time having to manually calculate XP values by referring to the tables in the core rules,thus reducing the need to look up info about a monster across multiple pages. The listed XP values are calculated directly from the standard monster XP tables.

See the common thread? None of these extras are new information. They simply collate existing information into each monster entry, to reduce the need to flip between different pages looking it up.

I agree that the use of the term THAC0 is open to discussion. I'm aware that it's a term that originated in AD&D, not in B/X, and I thought long and hard about whether to use it or not. In the end, my feeling was that it's a term that most old-school players are already familiar with, so would be less confusing than using a new term. Of course, the meaning of the term is described in the introduction to the book, as well (where it's specifically noted as optional), for the elucidation of anyone who's not familiar with it.

I appreciate that adding extra information to monster stat blocks is a change in presentation, but my feeling is that it's an extremely positive change that makes a real difference to using monster listings in the heat of (imaginary) combat. (Ditto for the changes in the way monster descriptions and special abilities are presented in B/X Essentials: Monsters.)

The really fundamental aim of the B/X Essentials project has always been to improve the presentation of the classic B/X rules. I'm not interested in changing the rules themselves, but I think my fresh presentation of them really adds something to the game.

Hopefully that helps to explain the background to this somewhat spiky, old-school ultra-nerd topic!

May those who like table lookups refer to tables, and may those who like subtraction use THAC0! (And may both types unite against the wicked forces of ascending AC! Haha.)


  1. It's kind of a shame that "To-Hit Un-Armored" wasn't the original standard instead of THAC0.

    1. Indeed.
      (Although I still prefer Target20)

    2. Would THAC9 be easier to use in some way? It's not something I've ever considered.

    3. Gavin, my thinking is just that you fight unarmored enemies all the time, and even most armored enemies have ACs that are closer to unarmored than they are to 0.

      So even if the math were all the same, there would be plenty of times you wouldn't need to do any math at all, and when you did have to calculate, you'd usually be dealing with smaller changes.

  2. Hi Gavin, nice work improving the monster's presentation. Btw, I've noticed that you omitted the relation between the monster level (that have a vague definition), its intelligence and the saves values. Any reason why?

    1. Hmm not sure what you mean there. A monster's level is its Hit Dice. B/X doesn't mention monster intelligence in any way. Could you explain more what you have in mind?

  3. About the saves...

    B30: "Unintelligent monsters save at 1/2 their monster level, with fractions rounded up. [...] Intelligent monsters usually save at their monster level."

    X27: "Save As gives the saving throw character class and level the monster saves at. Unintelligent monsters usually save at half their monster level, with fractions rounded up."

    It's usefull to estimate the saving throws when designing new monsters on the fly (:

    About the monster level, I agree. But the book contradicts itself sometimes. Like here:

    B28: "Silverleaf casts his spell and finds that 13 levels of monsters fall asleep. Since hobgoblins have 1 + 1 hit dice, they are treated as 2 hit die monsters for this purpose."

    And in the description of the Sleep spell

    B17: " 'Any pluses' are ignored (for example, 2 + 1 hit dice is treated as 2 hit dice). Creatures with less than 1 hit die are still considered as 1 hit die."

    1. Ohh that's really interesting! Do you know what? I'd totally missed that mention of the 1/2 level saves thing for unintelligent monsters! I'd noticed that there seemed to be a pattern of normal animal type monsters having these saves, but I didn't notice it's explicitly called out in that paragraph. That's kind of advice for designing monsters. I wonder if I can slip that in anywhere...

      Thanks for pointing this out!

  4. Here's the stat block for the people grumbling about B/X Essentials being incompatible with B/X.

    Armor Class: 4
    Hit Dice: 6 + 3*
    Move: 120' (40")
    2 claws/1 bite
    No. Appearing: 1-8(1-8)
    Save As: Fighter: 6
    Morale: 10 (8)
    Treasure Type: D
    Alignment: Chaotic

    1. Right, that's the standard B/X stat block, like the one for the doppleganger in the post.

    2. I was calling them Trolls. As in the internet variety.

    3. I guess it's like my wife says, I'm not as funny as I think I am.

  5. My concern is whether this breaks the modularity goal of B/X Essentials.

    For example, I can decide that monsters in my campaign are tougher and easily substitute the attack tables. Same goes for saving throws.

    I understand that the extra information is optional, but that leads me to another question: if a monster has a bonus to its to-hit rolls or saving throws, is that reflected in your statlines? Say some monster gets a +2 bonus to its saves vs poison, would you change SV P12 to SV P10, or just note the bonus in its description?

    This could lead to some confusion, whereas in the original B/X it's perfectly clear (it saves as a 1st-level fighter, but gets a +2 to its roll).

  6. Interesting point about modularity. I think it's not possible for _everything_ to be modular in that way, though. Certain core concepts are referred to throughout the books, so wouldn't be trivial to switch out. The books are designed to make the following things easy to switch out: classes, equipment, spells, monsters, magic items. Anything beyond that isn't any easier to house rule than it is with the standard B/X books, as far as I can see.

    About the monster saves, as far as I can remember, I didn't make modifications to the save values, but listed the bonuses in the descriptive text. If you can think of specific monsters that have a bonus like that (it's only a very few), we can check.

    1. The displacer beast comes to mind. It saves as F6 but receives a +2 bonus to all its saving throws.

    2. Good example. For that, BXE: Monsters explicitly notes the +2 to saves. (i.e. they're not pre-calculated into the save values.)

  7. Hell, I wish you would go the S&W route and add a bracketed AC structure to include Ascending AC. Anyway, PLEASE hurry with your plans for a single book/box set -I'll pay whatever you want :) Spiral bound would be an interesting option since it would then lay flat...

    1. Ascending AC is out of the question. That would be a pure rules _addition_, which is way outside of the scope of B/X Essentials. (Personally, I like ascending AC, though.)

      We're planning the combined book Kickstarter for early 2019. A few more months to go!