I have no real idea as to the history of Armor Class in the "alternate" combat system in OD&D... why it goes from 2 to 9, for example, rather than 1 to 8. We had a copy of chainmail, but never used the original combat system out of it. I only played OD&D for a year or two as a kid before AD&D came out, and we made the switch. When it came out, I embraced THAC0 as a great little innovation for simplifying things for players and DM alike.
Now that I've been running an OD&D campaign for about a year, and again using the combat charts from LBB 1 (Men and Magic), I find myself wanting to simplify things. THAC0? Nah. As I make changes to the rules, I'm trying to make them better support the gameworld I've created. In that world, normal humans are the dominant race, so the rules should be based on what normal humans do; with modifiers added as PCs level up. If I recall correctly, zero level men-at-arms attacked as 1st level fighters. I don't like that, since I've already added zero-level play to my rules. Here is the original Men Attacking chart:
And here is the Monsters Attacking chart:
Instead, add one to the first column in Men Attacking, and you get a required "to hit" roll of 18 for Plate Armor & Shield. That is what it takes a zero-level character to hit in my game. Therefore, make 18 the AC value rather than 2.
Now the chart looks like this:
We can see clearly here that a shield improves armor class by one. Should someone opt to use two shields, they won't be able to attack, but will improve their AC by two. In the same way, I'll allow a character to parry with his weapon instead of attacking, and that will also improve AC by one.
To account for the improvements gained when leveling up, we use a "combat bonus" based on the improvements in the Men Attacking table: +1 for levels 1-3, +3 for levels 4-6, +6 for level 7-9, +8 for levels 10-12, +10 for levels 13-15, and +13 for level 16 and above. Of course this is just for fighters. Clerics progress at 4 levels per group, and mages at 5 levels per group. Adventurers and thieves in my campaign fight as clerics, so here's the Combat Bonus Table:
To compare, lets look at a 5th level fighter attacking an opponent wearing chainmail & shield. Per the LLB chart above, we'd cross-reference the AC of Chainmail & Shield (AC=4) with the column labeled 4-6, and find that he needs to roll a 13 or better. Using my charts, we know that Chainmail & Shield is an AC of 16, and a 5th level fighter has a combat bonus of +3. Therefore, any roll of 13 or better would still be a hit.
To account for Monsters Attacking, their combat bonus will simply be their HD from now on, with any modifiers pushing them up to the next value, and 11 being the maximum bonus. This eliminates the need for the second table.
This won't be an exact match to the old tables, especially for monsters attacking, but I like the way it works, so as of today, that's how combat in my game will work. Hopefully, actual play-testing won't reveal some terrible bug I haven't thought of!