hey #dnd peeps, how would you explain THAC0 to somebody who never played before 3rd edition? isn’t it that thing with a crunchy or soft shell that you load up with meat and other toppings? It was a convoluted way of doing to-hit bonuses. The math worked out close to the same as 3e, but you went at it backwards.
Instead of having Base Attack Bonus, you had a THAC0 value you had to roll equal to or above, and the target's AC was subtracted from that value.
— Dan Dillon 👥 (@Dan_Dillon_1) December 29, 2019
THAC0 was the Benjamin-Franklin-establishing-the-positive-and-negative-charges-of-electricity of D&D:
It made sense at the time, but with hindsight proved totally backwards.