THAC0 was in use in 1E from very early days. A lot of the rules that grew into the various versions over the years started as "House Rules" Like Vince showed, it already existed in the 1E DMG appendices for Monsters, it just took players a while to figure out, "Hold on... I could use that exact same system for my character..." All you did was look at the combat matrix for your class, run your finger along to AC0, check your level, and THERE was your THAC0... For my group, it was about 1983 when we started putting a ToHt/AC:0 column on character sheets for every weapon so that we could include all the weapons' modifiers up front and know exactly what number we needed to roll every time we attacked. It became such a prevalent "House Rule" that it even reached Wisonsin, and was subsequently absorbed into the "Rules as Written" for 2E. When it was adapted into 2E it followed a structured progression that it hadn't in 1E, (of course it hadn't... it was 1E... what? you xpect it to make sense???) which is what allows macros like a Warrior's, 21-@{level}. Ta Da! I've been running a hybrid AD&D game for a while now, and among the first things I decided when using Roll20 was to go for the 2E THAC0. because its just mathematically simpler. I'm also in the process of migrating everything to go full 2E to take maximum advantage of Richards monumentally spectacularly brilliant RPGMaster set of Mods (APIs... whatever...) However much I tried to state how bloody good that suite is, I could not do it justice!