If there is a singular rule in any gaming system that I have always had trouble understanding despite knowing it, no matter how many times I revisit it or can use it, "To Hit Armor Class Zero" - THAC0 - is easily the most difficult. For those who do not know, the way to calculate the number needed to hit is the To Hit Armor Class Zero subtracted from the target's Armor Class, rather than just rolling a number, adding bonuses, and attempting to meet or best the Armor Class.