THAC0, in it's simplest explanation, is a calculation to determine whether an attack hits or misses in 2nd Edition Dungeons and Dragons (D&D). Read more about THAC0 in this excellent Reddit post detailing it's history. That post does it better justice than what I could write here.
Armor Class was a carry over wargames played by the creators for D&D; applying the "armor" of a ship or tank to the "armor" of a fantasy hero. A method was needed to determine whether an attack could breach that "armor". THAC0 was the answer and also a simplification of previous iterations of "to hit" tables from those wargames.
As a simplification it to it's wargames origins it made sense. Tables in the D&D books helped outline baselines across class and level. Math was not required as the expectation was that THAC0 was a value that was written on the character sheet and referenced against the tables. When rolling a D20 for an attack; players would know what they needed to hit.
The challenge comes in when you take into consideration the amount of shifting that occurs during a typical D&D encounter. The player character's values as well as the monster values were subject to constant change. Player's would receive buffs that increased attack strength or that changed armor class values for their target. With each change in number came a new change in THAC0 calculation. A good D&D group needed a proficient player that could calculate THAC0 reliably through any number of variable situations. Otherwise the session would bog down as pencil and paper were whipped out (a hard to imagine scenario with the current state of tablets and smart phones).
For my D&D group in high school I was the THAC0 calculator. My mind was built to focus on detailed rules like THAC0 and to ensure they were executed correctly. A major difference between classes in D&D was their calculated THAC0. It ensured priests were not going to be as effective combatants as warriors. Screwing up THAC0 calculations (often purposely) allowed classes to be the combat hero. Enforcing THAC0 correctly ensured that classes that were not meant to be hack'n'slash super stars relied on the other defining aspects of their "role". The benefit being better "role"playing.
My THAC0 calculator mindset extended into other aspects of the rules. I was known to crawl through class, weapon, and other rules to "keep the table honest" (as I was known to say). With that approach I became known as the "rules lawyer". That moniker followed me through numerous editions of D&D and even as THAC0 became a thing of the past and was replaced by much simpler "base attack bonus" modifiers I found other places to focus. Most recently with D&D 5E and playing with 8-12 year olds (father/son group) I've stepped right in to make sure the barbarian is using rage in every battle, that the bard is providing inspirations, that players are using that inspiration when applicable, or that our rogue isn't forgetting to apply advantage on attack rolls.
This rules lawyer (or alpha gaming) approach has been part of who I am as a gamer. It is likely evident to anyone that has followed this blog for any amount of time. While video games don't require someone to stay on top of all the calculations and rules there is always the analysis of what is best combination of things that result in the best outcome. I have the tendency to insert my opinion in those combinations from time to time.
It is hard to say whether having to be the THAC0 expert for my group made me into this style of gamer or whether I was destined to be this way. I would bet on the latter, but at the same time I can trace my gaming roots back to THAC0 as it influenced my view on where I fit into games. THAC0 made me the gamer I am today.
