Posts Tagged With: old-school

No, THAC0 Really Is Inferior

It pops up time and time again on D&D threads: is the ‘ascending AC’ system of D&D 3.x (where better defences = a higher number) superior to the ‘descending AC’ system of earlier editions of D&D (where better defences = a lower number, dropping into negatives eventually) and their legion of retroclones?

There are a lot of spurious arguments floating around, the primary one I keep seeing being that ‘addition is easier than subtraction’. That, and that having +1 chain mail that gives a -1 bonus to AC is counterintuitive, which I sort of buy – although I’ve played enough AD&D to know that you get used to the idea pretty quickly.

Anyway, I was thinking about this in the shower this morning and I think I’ve realised why I prefer ascending AC: it involves one less step in the calculation of each to-hit roll.

d20/ascending AC

  1. The player rolls the d20 and adds their bonuses.
  2. The GM recalls the target’s AC.
  3. The two numbers are compared. Hit or miss is established.

THAC0/descending AC

  1. The player rolls the d20 and adds their bonuses.
  2. The GM recalls the target’s AC.
  3. The AC is added to the d20 roll.
  4. The result is compared to the character’s THAC0. Hit or miss is established.


  1. The player rolls the d20 and adds their bonuses.
  2. The GM recalls the target’s AC.
  3. The to-hit table is consulted.
  4. The d20 roll and AC are cross-referenced on the table. Hit or miss is established.

It’s not much – just one extra step each time an attack roll is made – but it’s that little bit of extra arithmetic (or table consulting) that makes the THAC0 system less streamlined than the ascending AC system, and you make an awful lot of attack rolls in a typical┬ásession of D&D. As I’ve said, I’ve played enough AD&D to know that you can get used to it, but why should you have to?

That said, here’s a handy ascending AC conversion for Labyrinth Lord (which is my current retroclone of choice).

Attack Bonuses**

Level/HD Fighter/Dwarf/Elf/Halfling Cleric/Thief Magic-User Monster*
1 +1 +1 +1 +1
2 +1 +1 +1 +2
3 +2 +1 +1 +3
4 +3 +2 +2 +4
5 +4 +2 +2 +5
6 +5 +3 +2 +6
7 +6 +3 +2 +7
8 +6 +3 +3 +8
9 +7 +4 +3 +8
10 +8 +4 +3 +9
11 +8 +5 +4 +9
12 +9 +6 +4 +10
13 +10 +7 +5 +10
14 +11 +7 +6 +11
15 +12 +8 +6 +11
16 +13 +8 +7 +12
17 +14 +9 +7 +12
18 +15 +9 +7 +13
19 +16 +10 +8 +13
20 +16 +10 +8 +14
21 +16 +11 +9 +14
22 +16 +11 +9 +15
23 +16 +11 +9 +15
24+ +16 +11 +10 +15

0-level humans have an attack bonus of +0.

*Monsters with ‘X+Y’ HD attack as monsters of the next HD up. For example, a 1+1 HD monster attacks using the 2 HD bonus.

**It’s interesting, making this table, to note just how the numbers stack up. Monsters get murderous as their HD increase, being more likely to land a hit than a fighter at all levels up to 15 (and with more hp besides). Clerics, thieves and magic users all kind of suck by comparison although at low levels (where I assume most games happen) the differences are minimal. There’s no difference in attack rolls at all until level 3, so fighters have to rely on having more hp and better equipment until then.

To work out the AC bonus of a piece of armour, subtract the AC it offers from 10. For example leather armour (AC 8) becomes a +2 bonus, and chain mail (AC 5) becomes a +5 bonus. To make life easier, here’s a revised version of the armour table on page 15:

Armour Cost AC (desc.) AC bonus (asc.) Weight
Banded mail 85 gp 4 +6 35 lb
Chain mail 70 gp 5 +5 30 lb
Helmet 10 gp 5 lb
Horse barding 150 gp 5 +5 60 lb
Leather 6 gp 8 +2 15 lb
Padded 4 gp 8 +2 10 lb*
Plate mail 450 gp 3 +7 50 lb
Scale mail 50 gp 6 +4 40 lb
Shield 10 gp 1 less +1 10 lb
Splint mail 75 gp 4 +6 45 lb
Studded leather 30 gp 7 +3 20 lb
Unarmoured 9 +1

* Since padded armour is both cheaper and lighter than leather, and offers the same AC, I can’t imagine why anyone would ever wear leather. Perhaps it gives some sort of Cha bonus for being more stylish? Is it more durable?

Categories: Gaming, Theory | Tags: , , | Leave a comment

One-Page Dungeon Contest

A few years back, somebody in the old-school D&D movement came up with the bright idea of a one-page dungeon: a small map, some sketchy notes, and you’re good to go. This is the sort of neat concept you can have with old-school D&D games, since the stat block for any given monster is maybe three lines long and traps need no more detail than ‘anything put in a hole gets chopped off’.

And, over the past few years there has been an annual contest revolving around this: the One-Page Dungeon Contest. I entered a few years back, and was somewhat discouraged that my clearly superior* dungeon didn’t win any categories at all – I’m still quite pleased about the dungeon which serves no purpose other than to give its creator a chance to fall about laughing at the unfortunate fates of the PCs within.

Anyway, I thought I’d enter again this year, since I had an hour to kill over lunch. My entry is The Cult of the Tyrant Kings, and while it’s got it’s moments – Gavrik who thinks he’s a minotaur is my favourite, followed by Usebio and Lara who have a flat 50% chance of being a couple on any given day – I think it’s going to be roundly trounced by some of the competition I’ve seen. Oh well.

Still, it’s been a useful exercise in brevity – as the editors at Purple Duck can attest, I have a tendency to get verbose when describing dungeons. Having to cram everything (21 locations!) onto one page, even without any system info, is pretty hard. Even now I’m thinking that the Tyrant Kings have legs; hitting their cult should either be the culmination of a more investigative adventure or start you off on another trail, and it was difficult to hold myself back from filling in the details in the adventure itself.

If you feel like entering there’s still time! Only, like, 10 days, but since you can knock one of these out in *checks clock* about 1d3 hours, that’s not a great obstacle.

*Dungeon may not have actually been superior. =P

Categories: Gaming | Tags: | Leave a comment

Create a free website or blog at