It's a term from early editions of Dungeons & Dragons. Effectively, it was a rating of how good you were in combat, with lower numbers being better, 20 being the general "level 1 character" score - it represented the number you had to beat on a 20-sided-die
To
Hit a target of
Armor
Class
0 (AC ranged from -10 to 10). As you leveled up, your THAC0 went down.
In later editions, it was judged that this was obtuse and people are terrified of negative numbers (and certainly can't add them), so the whole THAC0 system was scrapped and replaced by the more well-known (now) "higher armor class is better" system.
THAC0 is and will always be the superior system. No matter what the mathematically challenged say.
Anyway, more importantly, I see the cookie count next in people's profiles, but I can't see the button to click.
There's a "give cookie" button under each person's avatar in a thread.
