[andrewas]

THACO - To Hit Armour Class Zero.

Its the number you need to roll on a D20 to hit a target with an armour of zero. If the target has another armour value, subtract it from your THACO and you get the number you need to roll. So if you have a THACO of 5 and your opponent has an AC of -2, you need to roll a 7, and will hit 70% of the time.

No matter what the THACO and AC are, you always miss on a natural 1, and always hit on a natural 20.