THac0

The correct POV

A simple system involving a base number (THac0) that you subtracted your die roll from.

So if your THac0 was 19, and you rolled a 12, you only hit AC 7.

Simple.

Really. Dumbasses.

An alternate POV

THAC0 stands for To Hit Armor Class 0 (zero). In 2nd edition AD&D, you subtracted the target's AC from your THAC0 and then tried to roll above the resulting number in order to hit. This meant that lower ACs were better, in contrast with most other stats. This inconsistency was obviously what made 2nd edition superior to all other editions with their regularized mechanics and simplified rules.