How Thac0 Existed in 1e AD&D

Thac0, otherwise known as To Hit Armor Class (AC) 0, was a combat system developed much before 2nd edition AD&D and was in fact actually developed by E. Gary Gygax as seen in an appendix of the Dungeon Masters Guide detailing all the very basic AD&D monster statistics. This however was not called “Thac0” directly but instead “To Hit AC 0”. So in thought, the system of Thac0 was not very new by the time of 2nd edition’s capitalization of the combat mechanic.