THAC0 explanation by BoneGrampa

This came in my email via r/adnd, and I thought I'd share it with you all. Back in the day, AC went down as it improved. That would be OD&D, 1E, 2E and Basic. I believe this to be a relic from historical and fantasy miniatures. Starting out with 3E, Wizards fixed this by using ascending AC and making it effectively your target number.

Back in the day, D&D was very big on tables -- many, many tables for most everything, including to hit and saves. You used to have to consult a table for your class or for the monster's HD and run your finger across to the correct AC to read what the target number was.

What the introduction THAC0 as a concept and a thing is that it revealed the underlying logic of the to hit tables. In actuality, everything was really calculated from AC 0. If you knew your class or monster THAC0 for level or HD, you could calculate and recreate the table yourself. (My personal D&D character sheets tended to be on quadrille graph paper for exactly this reason.)

Ye Olde Grognards tend to wax nostalgic about THAC0, not because it's better than what we currently have (it's not), but because it was the BOMB for hacking the system back-in-the-day when we were playing.

This an interesting way to explaining THAC0 to beginners, backwards to how we actually did it then. But it's equivalent and maybe not a bad place to start from. I love the homemade art that accompanies it -- very reminiscent of Basic and 1E art specifically, which I am still very fond of and inspired by even today!