So a friend of mine wants to run an oldskool ADnD 2nd edition game. I am trying to get a handle on the rules...

So a friend of mine wants to run an oldskool ADnD 2nd edition game. I am trying to get a handle on the rules. How does THAC0 work?

First level. Thac0 is 20. Roll d20 to hit. Count UP from the number you rolled to 20. That is the AC you hit.

Each level you might have to change that number based on class. Worry about that later.

Same with magical weapons. Worry about it later, just get your head round the basics. Not difficult.

It's basically just the inversion of the system as you know. You subtract AC to it to find the TN. So THACO 15 and AC 9? You need 6 or more to hit.

THAC0 stands for "To Hit Armor Class 0". Make your attack roll, and add the target's armor class to it. If the sum is greater than or equal to your THAC0, it's a hit.

Example
>first level
Roll 15 on d20
>hit AC 5

>Third level fighter
Thac0 18
>roll 15 on d20
Hit AC 3
Etc etc

Is THAC0 "good?" Because I've seen even the Retroclones replace this system.

It's a little non-intuitive to new players. Not inherently bad.

Die roll - THAC0 = AC you can hit.

THAC0 differs by class and level but your DM should just tell you it.

It isn't "bad", It's just difficult to explain to retards, most systems that replace it are functionally identical.

OP the basic rule is: :THAC0 − AC = roll needed to hit.

So Lets take the example of a Generic Adventurer and Magical Training Dummy.

The Adventurer has THAC0 20, AC -1. The Magical Training Dummy has THAC0 15, AC 10.

The Adventurer attacks the dummy. He Checks his THAC0 (20) and Subtracts the AC of the Dummy (10). Plugging in the numbers to the rule we get: 20-10 = 10. The adventurer needs a roll a total of 10 or more on the D20 hit roll to hit the Dummy.

The Dummy then attacks the Adventurer. He checks using his numbers. In this case THAC0 (15) vs AC (-1). Plug in the numbers to the rule and we get: 15 - (-1) = 16. For simplicity, and because some people don't understand negative negatives, treat all "negative armour classes" as positives: So 15+1 = 16. The dummy needs to roll a 16 or more to hit the adventurer.