Okay, when D&D first came out, they used a 20 sided die. Now this is still used internally in the game BG. Say you have a THAC0 of 15, that means that you need to roll a 15 on that 20 sided die to hit AC 0. Say you're THAC0 was 10, then you'd only need a 10 to hit AC 0. If you're THAC0 was 12 and your enemies' AC was 4, you would need to roll an 8 on that 20 sided die. Understand?
------------------
Formidable Flirty Fishy Sorcerer of the HADB