LogFAQs > #944946467

To go into more detail, in ADnD 2nd edition you wanted to roll as high as possible to hit your enemy. However the lower the armour class your enemy has the higher the result you would need to roll. In order to save on maths players would calculate what number they would need to roll in order to hit an enemy with an armour class of zero, which was at the started of ADnD2 was the highest (lowest) possible armour class. If you roll a dice and had a score above your Thac0 you knew you hit your target, if you had less than that you would look at the difference between your roll and Thac0 as well as your enemies armour class above zero. It was pretty complex and part of the reason for the major changes in DnD 3rd edition.