Chainmail: Rules for Medieval Miniatures 1971

Chainmail is a medieval miniatures wargame created by Gary Gygax and fellow Lake Geneva Tactical Studies Association (LGTSA) member Jeff Perren, a hobby-shop owner with whom Gygax had become friendly. The game was first published in 1971 by Guidon Games. That first edition also included a fantasy supplement, and is one of the oldest sets of rules for fantasy miniature wargaming.

In 1975, Tactical Studies Rules (TSR) acquired the rights to publish Chainmail and the game may have been the basis for the earliest Dungeons & Dragons game and campaigns (Dave Arneson, co-author of the original D&D game, apparently disputes this). In the original boxed set of D&D one is directed to Chainmail for the combat system but it offered an "optional" combat system involving 20 sided dice -- a system that eventually became the D&D standard.

Regardless of who you believe, the Chainmail fantasy supplement contained spells and monsters that reappeared in Dungeons & Dragons and most certainly influenced Gary Gygax and his thinking about Fantasy games in that era.