The Way to Do Ascending Armor Class

Why Ascending?

As a DM who loves the OSR, you probably want to get as many people into it as possible. Thus, if there is something you can do to make the game more accessible without any functional changes, you should probably do it. For example: using ascending armor class, which the vast majority of players are more familiar with than descending armor class.

Of course, old adventure modules you will use at the table descending armor class. In this case, it's easy for you, as the DM, to look at the page and convert the number in your head. Far easier and faster, at least, than having to explain attack tables or THAC0 to each new player.

The Full Score

Conversion is as simple as subtracting the old AC or THAC0 value from your conversion base, which is a fixed number. So what do you choose as this base? If the choice can make the mental arithmetic even a tiny sliver easier for you, then it will save a lot of time in the long run. I find that a round number like 20 is slightly easier to subtract from.

So your conversion formula would look like this:

Desceding AC = 20 - Ascending AC
Attack Bonus = 20 - THAC0

Thus, if you're reading an old module and see a monster with 5 AC, you tell the players that it has 15 AC. Monster attack bonus is an even easier case: you don't even need to check the attack table if you can memorize how monster HD relates to attack bonus. In our case, that is exceptionally easy - monsters have an attack bonus equal to their HD! (At least up to 8 HD - the B/X attack progression slows down at very high HDs.) That means that monsters with less than 1 HD have +1, and so on. "Normal men", which I like to think of as "level 0 types", have an attack bonus of +0.

Very clean, right? It's kind of intuitive that all 1st level PCs attack at +1, and thus all "normal humans" - those who haven't gained a class - attack at +0. Anyone with a class attacks at +1 (you can think of 'monster' as a class).

If you chose another number as your base, say 19, then level 0 types would attack at -1? That's weird.

So if 20 is such a good base, surely most modern AAC-using retroclones use it? As it turns out, no.

The Big Comparison

Swords & Wizardry offers both descending and ascending AC. Old-school descending AC and attack tables are the same as B/X - unarmored AC is 9. In ascending AC, unarmored is 10. This means that to hit an opponent with 20 AC, a 1st level Fighter needs to roll a 20 - as if her attack bonus was +0. So does that mean normal humans have an attack bonus of -1? The book doesn't say. Is the authors's sole reason for choosing 19 as the conversion base that an "unarmored base AC" of 10 looks pretty? If so, I don't think that's a very good reason.

B/X Essentials only includes descending armor class, but its upcoming refinement Old-School Essentials adds an option for ascending armor class. It sets unarmored AC at 10, taking its cue from Swords & Wizardry.

Lamentations of the Flame Princess goes in a completely different direction. There, unarmored characters have 12 AC (ascending). In other words, the conversion base is 21 (21-9 = 12). To match, Fighters attack at +2. But all other PC classes only attack at +1, so they've been demoted to "normal humans" - while NPCs themselves attack at +0! Overall, everything is harder to hit, to suit the game's greater emphasis on horror. I actually think this is a bit of a trap for new players, who get recommended LotFP as the "default, generic" OSR game, but are faced with a lot more frustration than anticipated. Buyer beware!

Basic Fantasy RPG, for one, does it right. PCs have an attack bonus of +1, normal men of +0, and unarmored AC is 11! Hooray! For everything BFRPG does that I disagree with, here is one part where they knock it out of the park: designing with the actual use case (on-the-fly conversion) in mind, rather than for a surface appearance of elegance.

I don't think anyone cares if unarmored AC is a nice round number - when it goes on the sheet, it gets modified by their equipment and Dexterity anyway. Basic Fantasy: 1, Swords & Wizardry: 0.

Of course, if you use the above system and play a S&W module that lists both DAC and AAC, you will need to either use the DAC for conversion, raise the quoted AAC by 1, or use the AAC as-is and be happy with the fact that monsters are nerfed by 1 AC, which is probably perfectly fine.

(Another tiny caveat: the above only talks about converting B/X and OD&D materials, where unarmored is 9 and Fighters have THAC0 19. AD&D actually changes these, moving unarmored to 10 and giving Fighters THAC0 20 to match. If you use the above to convert the AC values from an AD&D module, you will end up nerfing the monsters by 1 point. And that's probably perfectly fine too.)

Simple OSR Thief skills on a d20

Observation 1: Most of the Thief skills in B/X D&D improve in increments of 5%, which makes using a percentile die rather pointless. You can take the percentile (and x-in-6) Thief skills table, convert all the values onto a d20 target number, and it looks like this (TN rounded up):

Level	Climb Sheer Surfaces	Find/Remove Traps	Hear Noise	Hide In Shadows	Move Silently	Pick Locks	Pick Pockets
1	3	18	14	18	16	17	16
2	3	17	14	17	15	16	15
3	3	16	10	16	14	15	14
4	2	15	10	15	13	14	13
5	2	14	10	14	12	13	12
6	2	12	10	13	11	11	11
7	2	10	7	11	9	9	9
8	2	8	7	9	7	7	7
9	1	6	7	7	5	5	5
10	1	4	7	5	3	3	3
11	1	2	4	3	1	1	1
12	1	1	4	2	1	1	0
13	1	1	4	1	1	1	0
14	1	1	4	1	1	1	0

Observation 2: From this table it is easy to see that most of the skills - Find/Remove Traps, Hide in Shadows, Move Silently, Pick Locks and Pick Pockets - are always within a band of 2 points of each other. Generally, they start at 16-18, improve 1 point per level, before accelerating to a rate of 2 points per level starting at 5th or 6th level. These could very easily be collapsed into one chance without losing much precision - let’s say into the middle chance, Pick Locks, a 17.

One of the remaining two skills, Hear Noise is one that many tables including mine (and some retroclone rulesets) ignore, simply letting players succeed to listen if they spend a bit of time, so it can be ignored here. Climb Sheer Walls is a bit of a tricky one. It’s the only one that starts at a high chance of success, 87%, and improves by one percentage point per level, so it ill fits non-percentile models. However, there is precedent in the OSR of merging Climb into a more “unified” skill system, in LotFP, where Climb improves in steps of the same size as other skills. If Specialists put all their points at 1st level towards Climb, they end up with chances quite similar to the ones in B/X - good chances in climbing (5/6), terrible chances of doing anything else (1/6).

Hacking it

Let’s assume that in our modern mindset we want a resolution mechanic that can be applied not just to Thieves, but all classes, while still retaining Thief superiority. Naturally, we achieve that by treating all other characters as zeroth level Thieves. So, ignoring Climb for a moment, we choose a 17 as our target number at 1st level, increasing by one each level. In other words, to succeed at a Thief skill, you must roll d20 + Thief level for a result of 18 or higher.

Hell, why not call it a nice round easy-to-remember 15, and give Thieves a little boost? The B/X Thief is commonly seen as too inefficient, and in fact AD&D boosts many of the thieving chances by 10%. Now, doing this does improve all characters’ odds from the default 16.6% of LotFP to 30%. But, especially these days, most OSR play takes place in the low levels. I’m okay with letting characters do a bit more. (For comparison, the Mutants & Magic proposal linked above lets 1st-level Thieves succeed on most things on a 12-14+.) Incidentally, choosing 15 as the target number, the new chances start to fall behind the B/X Thief's at 8th-9th level.

"Most LotFP campaigns feature lower level characters, and modern campaigns do not last long enough to build characters to higher levels. ... providing characters more possibilities at lower level and decreasing the importance of gaining levels is important."

- James Raggi, "Playtest Notes" in Eldritch Cock

Now that we’ve moved all skills to this mechanic, the obvious next step is other “common activities” with X-in-6 odds like Foraging, which LotFP turns into the Bushcraft skill, and Open Doors, which is not a skill but an X-in-6 chance modified by Strength. Open Doors should certainly be modified by Strength in our new system too. One benefit of moving to the d20 is that if you want ability scores to affect the odds of skills, you can, simply by adding the ability modifier to the roll. Wisdom would be used for Bushcraft, Dexterity for Stealth, Intelligence for Tinker, and so on. The modifiers have a bit of an impact but do not overflow the die size. Also, we are now rolling high (like all other rolls), instead of rolling low on a d6 or percentile, if you care about that sort of unification/elegance nonsense. (I don't think the d6 is bad at all, personally, in fact I really like how chunky the increments feel.)

Finally, let’s get suspiciously modern and use the term “proficiency” for adding Thief levels to the roll. Thieves shall have proficiency in Stealth, Climb, and Tinker. If we take a leaf from LotFP’s increasing demihuman skills, then Dwarves would have proficiency with Architecture, Elves with Search, and Halflings with Bushcraft (and also have a +10 to hide in the wilderness). I would also probably give Fighters proficiency in Open Doors, because why not? They’re likely to be the most physical type.

A summary of the skill system

All activities and skills (Climbing Sheer Walls, Finding Traps, Removing Traps, Hearing Noise, Hiding in Shadows, Moving Silently, Opening Locks, Picking Pockets, Forcing Open Doors) are rolled on a d20. The appropriate ability modifier is added, as well as Thief level if the activity is thiefy. A result of 15+ is a success.

Skills/activities:

• Architecture: INT, +Dwarf level
• Bushcraft: WIS, +Halfling level
• Climb: STR, +Thief level
• Languages, Medicine: INT
• Open Doors: STR, +Fighter level
• Search: INT, +Elf level
• Stealth, Sleight of Hand: DEX, +Thief level (these skills can be rolled into one)
• Tinker: INT, +Thief level

(Sneak attack is no longer a skill, but a simple 2x multiplier, possibly increasing with Thief level.)

Now that all Thief activities progress the same, you can combine those that use the same ability, i.e. Stealth and Sleight of Hand. If you see Tinker (Pick/Remove Traps) as an activity requiring more dexterity than intelligence, then that can be merged in too for a general "Thievery" skill. As an extension of the system, you could have character backgrounds that give you background skills granting a +2 bonus to certain activities.

Pros & Cons

So what’s the score? We have made abilities slightly more important, which depending on your taste may be a good thing or a bad thing (but actually less important compared to roll-under ability checks). We now have a Thief and not a Specialist - we’ve removed “builds”, option paralysis, customization - which again you may see as either good or bad. We've made Thieves decent at everything, but far worse at climbing (I dunno, give them a +10 to climb or something if you want). As a positive, we’ve reduced the space taken up by skills on the character sheet, since you no longer need to list the individual skill levels, only whether you’re proficient in them - adding your level to the roll is easy enough. And there's no need to print a Thief skills table in the rulebook anymore.

However, I see some serious risks in going in this direction. Those risks are psychological shifts that may happen in dungeon masters when moving from a “X in Y” chance to a target number which has treacherous resemblance to the modern “d20 + proficiency vs. difficulty class” mechanic of 5e. The danger is that DMs will start to see the fixed target number of 15 as a "default DC" that they will begin to carelessly modify. They may be tempted to do this where they would not have been in a d6 skill system. LotFP's rules only mention modification of the chances in two places: opening giant stone doors, and attempting to forage in different terrains such as the desert. Modifying the target numbers too often may give rise to a treadmill where characters get more competent but DMs constantly raise the obstacles in response. Because... for some reason a locked chest in a high-level dungeon would have a higher DC to open? I am generally against that, and it should be used very sparingly, as LotFP does. Just let players know their chances of success, and let characters feel competent! This psychological risk of treadmills could perhaps be mitigated by pre-applying the modifiers and listing the naked die result required on the character sheet, as saving throws are handled, though that would take up a bit more space on the sheet and require updating every level.

The other risk is that it will tempt DMs to call for knowledge rolls, and perception rolls. Those are harmful too, but the explanation why is a subject for another blog.

Finally, I'd like to say that I'm not under the impression any of this is an original idea. The Thief seems to be one of those topics of eternal discussion. In fact I'm almost certain there are already several blogposts with same idea, but Google failed to find me any so I'm barfing my notes on here.

OSR Fighters: Combining Cleave and Extra Attack

You’re probably aware of the multi-attack abilities fighters have had over the years.  There’s the “Melee attacks per round” table, where Fighters in AD&D could have 1/1, 3/2 or 2/1 attacks per round. That fractional value of 3/2 means your number of attacks alternates each round: twice in one round, then once in the next, and so on. Sure, it’s not hard to understand - but it’s still an additional thing to remember in the heat of battle, which I don’t want in my game. Newer editions call this Extra Attack, but omit the fractions.

Then there’s the “one attack per level against creatures with one/less than one hit die”, originating from Heroes in Chainmail having the fighting ability of four men. This rule is sometimes called "cleave". The same ability is also pointed at underneath the “Melee attacks per round” table in AD&D, referring mysteriously to the COMBAT section, which offers no explanation on the matter.

A rather popular rule for “cleaving” weak foes that doesn’t require you to track the number of attacks is what I have just decided to call “If They’re Slain, Swing Again”. It grants a free attack each time you slay an enemy, continuing as long as you either fail to score a kill or there are no more enemies within reach. The less hit dice the opponents have, the more likely you are to get multiple attacks off.

Here is yet another option I came up with which combines aspects of both extra attacks and cleaves:

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Ability: Fighter's Focus

If you hit an enemy with an attack on your turn, you can attack one more time. If this second attack is a miss or a non-killing blow, your turn then ends. However, if it is a killing blow, you can attack again, and keep attacking as long as you keep landing killing blows, or until there are no more targets within range.

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Rationale

The condition of Fighter's Focus means that you get a sort-of-an-extra-attack, but not always - only when the first blow lands. It’s like gaining momentum in the melee. Then on the second blow, you really get into the zone and potentially start mowing everything down. Perhaps you could give these two "modes" of focus different names to help players remember how they work. (If I have to draw a flowchart, or nobody remembers how it works, then it must be a bad rule.) It also means that you don't need to keep track your alternating rounds like in AD&D.

Obviously, the second stage of focus still works best against low-HD creatures, while the first part gives more of an advantage against mushy enemies with poor AC. Thus it will sometimes give you an edge against high-HD creatures, too. All in all, the ability should end up being less powerful than straight up giving Fighters two attacks, but more powerful than the “If They’re Slain, Swing Again” rule by itself. It’s somewhere in between.

I’ve been exploring these different options recently since I’m looking to change some rules for the next game I run. Specifically, breaking from LotFP, I want to give non-Fighters the increasing to-hit that they get in B/X. But I still want Dwarves and Elves (and Paladins etc. that I may add) to have the improved combat options, as they do in LotFP, and have no equipment restrictions. So with Fighters losing their only niche, what do you give them so they don’t fall behind? Well, more attacks, of course.