Talk:THAC0/@comment-11205547-20171115030204/@comment-11141165-20180126162942

tl;dr: Simplified formula for hitting an opponent is

[your THAC0] - [enemy's AC] = [how high you need to roll on a d20].

In the pen and paper D&D, when you try to hit a creature with your weapon you roll a 20 sided die. Now you roll it and a number comes up. So do you hit or not? Well, there needs to be some way to tell how high you need to roll to hit it, right?

Your THAC0 and your enemy's Armor Class (AC) is what tells you how high a number you need to get on your die roll. Let's say your enemy has AC 0. If you are first level your THAC0 is 20, which means you need to roll a 20 on the die to hit it (it is called To Hit Armor Class 0, after all). If the creature's AC is 10, then you need 20 - 10 = 10 on your die roll (or higher) to hit. The formula is [your THAC0] - [enemy's AC] = [how high you need to roll]. If your THAC0 is 0, then you will always hit an enemy with an AC of 0 because 0 - 0 = 0 (except that a roll of 1 on the die is always a critical miss in any circumstance).

Then there are adjustments to THAC0 like for example a high Strength score or a magical weapon. Those are very straight forward, but it means that with a Strength score of 17, your 1st level character can hit the AC 0 enemy with a die roll of 19 or 20, i.e. needs one lower on the die roll to hit it.