Weapon vs AC "To Hit" Adjustments and the "hit proof" Target Problem

It's likely that anyone interested enough to read this already has a better grasp on the the subject that I did when returning to AD&D 1e, but I hope it proves useful to you. After a very long time of thinking I understood these rules, I finally believe I really do understand how they are supposed to work in AD&D 1e. Let's start with some tables:

Combat table for Fighters DMG pg. 74

Although not coming until 8 pages later, the "Progression on the Combat Tables" does a good job of explaining the otherwise odd looking arrangement of 20s in the table; they are there to provide a threshold to "impossible to hit" targets.

DMG pg. 82

I don't really have an opinion on which way to go with the natural/adjusted rolls at this stage, my gut feeling is to allow adjusted results unless the target has some particular reason not to.

DMG pg. 70

Though it probably could be located closer to the other two sections in the DMG, this paragraph is crucial; without this rule, if I need 20 to hit and I have a net -1 penalty then I can never hit! Instead though you should consult the combat table and shift the AC of the target up one place, a result of a 20 means you can still hit on a roll that high. This help does not always make a difference; a -6 penalty will put it into the range were the target cannot be hit, but by this stage that is a reasonable thing given the power imbalance indicated. When I first re-read this coming back from remembering 2e THACO, I couldn't make sense of it; because with THACO one step either way works more or less the same. In 1e this is the difference; penalties have a mitigated impact while bonuses do not. 

PHB pg. 38

Now, look at the penalties in the To Hit Adjustments chart; in the same way as described above for "certain spells" these too are not modifications to attack rolls, they are modifications to the Armour Class of the target. 

Consider a 1st level fighter with a battle axe vs an enemy in platemail and shield (AC 2). The fighter can hit AC 2 on an 18, but the weapon vs AC penalty of -3 if applied to the roll would make it impossible for him to hit; but, moving up the AC by 3 on the Combat Table results in a To Hit of 20; he can still do it. Contrast him with a level 1 fighter with a footman's flail; he too has a To Hit of 18, but he gets a +2 bonus and can now hit the enemy on a roll of 16. Both fighters can hit, but the one with the foresight to bring an alternate weapon reaps the reward. Similar trade offs can be made at other levels of AC.

Why is it so?

The paragraphs from the DMG pretty well explain the reasoning; it's about when and how a target should be "hit proof"; but, I would extend this explanation by comparison with later releases. 2e does away with the combat table for the much maligned THACO system, so effective was this change that many forget that THACO was not a 1e thing at all. 2e also introduced the rule that a roll of 1 is always a miss and a roll of 20 is always a hit (still the case in 5e). I think this dumbs things down and ignores the problem 1e was trying to deal with by making a broader threshold before a target is hit proof. 2e also simplified and made optional Weapon Type vs AC adjustment. 

This combination of rule changes reduces player agency in weapon choice, and also, no minor thing, scales poorly in mass combat. In mass battle 40 Goblins with bows will, on average, hit a fighter with AC -10 (or whatever that is in ascending AC, yuck) twice a round under the simpler system and not at all in 1e. 

The 1e solution to a "hit proof" target makes the threshold more difficult, but not impossible, to reach; which I think is superior to its successors. Like many rules in AD&D it might not be a totally accurate representation of the "real world", and you might go a long time without it being relevant; but, when the situation does come up the rules work well and do what they were intended to do.

If you've read this far you might be interested in what Nagora did to put all the numbers in one place on the character sheet.

The Combat Wheel

Of course, the ultimate instantiation of the Combat tables is in the Combat Wheel. I've lost track of where I sourced these from so I can't attribute them (but thanks to who ever it was). I sent these to a printer for laminating and it worked quite well.