Revisting an old problem: THAC0 WTF?

[Tsotha-lanti]

It's incredibly simple.

THAC0 is "to hit armor class 0." It's the minimum number you need to roll, on a d20, to hit AC 0. To hit a specific AC, you subtract the AC from the THAC0.

So if your THAC0 is 20, and the AC is 8, you need to roll 20-8=12.

If your THAC0 is 13, and the AC is -4, you need to roll 13-(-4)=17.

Thus, high (positive) numbers are bad, low (negative) numbers are good, for AC. A high number is bad for THAC0, and a low number is good.

3.X got rid of it, and made the system even more simple (and more intuitive): rolls are now d20 + bonus against a target number; your result must equal or exceed that target number.

Edit: To be more specific to your question, the difference is that AC is a defensive capability, and THAC0 is an offensive capability.