Simplifying THAC0 (and attack matrices)

I usually prefer ascending AC to descending AC, but I never thought THAC0 was particularly complicated.

You don't even need subtraction to use THAC0. 

Just roll 1d20, add AC and modifiers, and beat THAC0. Done. 

Delta has already done most of the work. I also found this post by @contrarian, which inspired most of what I'm writing here - and provided most of the images.

One thing I dislike are attack matrices, because I feel there is no need to check a table for that. If you need a "to hit" number, a single digit will do.

With that said, there ARE some interesting aspects of the AD&D matrices. But they could easily be reduced to this:

* Roll 1d20+AC+modifiers.

* Add your level if you're a fighter-type, half your level if you aren't.

* If you roll 20 or more, you hit.

And... that is it, basically. 

We still have to deal with negative AC, but we'll get there.

Let's see. This is the original DMG table:

Notice that 20s are repeated SIX TIMES, making many kinds of armor practically identical. 

This six-point spread is huge; in OD&D and B/X, for example, it is as big as the difference between UNARMORED and PLATE!

Here is an optional rule that allows you to roll higher than 20 on the d20, but requires a natural 20.

I dislike this table because it makes negative AC too powerful against a fighter of amazing strength and magic weapons. And STILL treats AC -2 identically to AC -7 in some cases.

However, the idea that you ALWAYS have a small chance to hit - even negative AC - is nice.

How often do your 1st-level PCs fight monsters with negative AC? Not often. 

BUT: I'm guessing the multiple 20s are there to allow, say, an army of NPC archers to fight a dragon, which makes sense.

How can we achieve the same effect without the tables? Well, you could simply add +5 to your roll if you hit AC 0 BEFORE considering the negative AC. 

But, again, this makes many types of different armor identical in practice.

So, my idea is that negative AC is not added to your roll (count it as zero), but subtracted from your damage. 

Maybe a powerful attacker can CHOOSE between subtracting AC from the roll or from the damage (sometimes, you NEED to take the "less damage" option).

Now AC 0 is different from -1 or -2, etc.

Gygax considered something vaguely similar is Isle of the Ape.

(Of course, we could go even further. Since you need 20 or more to hit, each point over 20 could be translated to damage (something that AD&D also suggests for fighter that hit automatically). But now we are getting further away from the original. Anyway...)

Here are a few additional considerations:

- Should clerics, thieves, and MUs use the same table? 

Yes. Clerics are too tough and thieves too weak already. For MUs, if really doesn't make a difference - your level 10th MU will use its 10d6 fireball rather than 1d4 dagger.

- But shouldn't a MU keep the same attacking capabilities until level 6? 

Not really. Gygax indicates a smoother curve for Fighters - why not do the same for other classes?

- What about backstabbing?

Since this is mentioned in the matrix, I will suggest thieves/assassins simply add their level to backstabbing damage (maybe a minimum if +4). This encourages them to attack with small weapons, and gives them a little boost every level. The books indicate that the only part of the damage that is multiplied is the weapon dice, not Str or magical bonuses. 

So, a thief with a magical shortsword (say, 1d6+2) dealing quintuple damage would roll 5d6+2. This is about 19.5 on average. 1d6+19 is just slightly higher than that, and the thief deserves the boost.

- What about monsters?

Treat them as fighters. Much easier but not that much different.

- But I want THAC0!

Here you go. Expand to level 20 or whatever you want.

- But I want something EVEN CLOSER to the original matrices!

In that case, check the original post by @contrarian. Great stuff!

Also, let me know about any other objections to my solutions!