E-Diction War

THAC0 or “to hit armour class 0” was the kludge way 2nd Edition got around having to have charts and tables determine if an attack roll was successful, using some reverse addition and funky math. So if you had a THAC0 of 11 and rolled a “13” with a +2 bonus, you’d hit AC -5, which was better than AC 0 or AC 5. But if you had a THAC0 of 18 and rolled the same, you’d only have hit AC 3.

While not *that* much harder in terms of math, it was noticeably slower at the table. You had to total up your bonuses and then do the math to figure out what AC you would have hit to relay that info to your DM. While in 3e and beyond, just totalling up your bonuses was all the required math. (Although, the bonuses tended to get a lot higher.)