There are many varying opinions on THAC0 (To Hit Armor Class Zero), the combat system used by 2nd Edition AD&D (Advanced Dungeons and Dragons). Many people seem confused by it and therefore don’t like it. They find it to be counterintuitive.
Using the THAC0 system, each character has their own THAC0, roughly representing their overall combat ability. To figure what number you need to roll in order to hit a particular target, you take the target’s AC (Armor Class) and subtract it from your character’s THAC0. Lower ACs are better than higher. This is where people get confused and find it to be unintuitive, because you end up subtracting positive ACs from your THAC0 and adding negative ACs to your THAC0.
But there is an easier way.
Instead of adjusting the character’s THAC0, leave their THAC0 where it is and add the target’s AC to your to-hit roll instead.
For instance, let’s say your character has a THAC0 of 20. Now let’s say you’re facing a target with an AC of 5. Make your to-hit roll, then add 5 to the roll. If the result is 20 or higher, you’ve hit.
Now let’s say you’re facing a target with an AC of -5. Again make your to-hit roll, and this time subtract 5 from your roll. You would still need a result of 20 or higher to hit, which in this instance would be a pretty neat trick.
By having ACs affect the roll instead of having them affect THAC0, using THAC0 becomes much faster and easier to understand.