I Promise, Descending AC Is Easy, and Good.
Let's say you've been given a voucher for a free flight, any ticket on the plane. There's no question that you'll usually want first class, right? The high class seat is number one in quality; the highest rank is usually number one. With me so far? Okay, awesome.
ThAC0, besides being fun to say, shouldn't be intimidating. It just means, what will I need to roll to hit Armour Class 0. That's a Fighter in plate mail with an 18 DEX. High class! Top of their game, this one - and yes, you can be dexterous in plate mail, I promise.
Now, in contrast, being unarmoured and not particularly athletic (say, DEX of 9-12) means your Armour Class is 9. Your relative (or perhaps literal, it's your character) nudity essentially grants your opponent a +9 bonus to wound you, compared to our AC 0 friend. Higher class armour, lower number, harder to kill.
The elegant thing about this, math-wise, is that this particularly stout opponent is only struck on a 19+; on a d20, a new character of modest skill usually only has a 10% chance of wounding an AC 0 opponent. An AC 9 opponent, meanwhile, is wounded on a 10+, a 55% chance - better than average!
Following along? Yeah? Sorta? Here's another, less formulaic way to think of it. You want to hit an AC 0. That's your goal. This starts at 19+, and gets easier when you level. We're just gonna focus on 19.
First, since we're a fighter using a sword we add our strength modifier. We're STR 14, pretty good! That's a +1. Our sword is a +1 magical sword, so that's +2 so far. Awesome! Now, what's our opponent's armour. They're wearing leather, and sporting a shield? Why, that's AC 6, so let's add another +6 to our roll. That's 8 total.
19 (our magic number) - 8 (all our bonuses, or their penalties if you prefer) = 11. We need to roll an 11 or better.
Even if you have STR 4 and they're in plate mail with high DEX and a +1 shield, a 20 still catches em, so you have a shot at wounding; and no matter how spectacular your talents and equipment, you can still whiff on a 1, so there's a chance of a miss.
Of course, that's assuming you need to roll, which you probably don't. I'll talk about that next week.