Chainmail was a set of miniature rules for Medieval / Fantasy battles. Depending on who you believe, it may have been the basis for early D&D campaigns (Arneson 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.
Whoever you believe, Chainmail was written by Jeff Perren and revised and expanded by Gary Gygax and certainly influenced his thinking about Fantasy games in that era.
