From the AD&D 2nd Edition player's handbook: THAC0 is an acronym for "To Hit Armor Class 0." This is the number a character, NPC, or monster needs to attack an Armor Class 0 target successfully. THAC0 depends on a character's group and level (see Table 53 on page 91). The THAC0 number can be used to calculate the number needed to hit any Armor Class. THAC0 is referred each time a character increases in level. Using THAC0 speeds the play of combat greatly.

It's #THAC0Thursday! Get out our old school rpgs and represent.

A sampling of the D&D ads that used to run in comic books in the 80s. Sometimes these grabbed my imagination more than the comic they ran in. Fantastic adventure panel by panel... on THAC0 Thursday!!
#thac0thursday #thacothursday #dnd #adnd #rpg #ttrpg #tsr #comics #comicbooks

"My Life as a Werebear: D&D Monster Character Classes" from White Dwarf #17 (1980) by Lewis Pulsipher & art by Tom Wham discussed using monsters as player characters. You can really pack in the adventure for a howling good time... on #THAC0Thursday!!
H/t oldschoolfrp
#dnd #rpg #ttrpg #80s #1980s

Babysitters? We don't need no steenkin' babysitters... on THAC0 Thursday!!
#thac0thursday #thacothursday #dnd #rpg #games #1980s #80s

A 1980 TSR memo from Gary Gygax to Steve Marsh outlining the criteria for the various types of planes and how difficult it might be for a player to catch a flight to one. When can a plane be a pocket? On THAC0 Thursday!!
#thac0thursday #thacothursday #dnd #rpg #TSR #history #garygygax

LJN Toys Ltd. released this bad boy (the only playset sold in their AD&D toy line) under license with TSR in 1983, complete with traps and a flying throne. Fangs for the memories... on THAC0 Thursday!!
#thac0thursday #thacothursday #dnd #adnd #rpg #ttrpg #toys #retrotoys