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 - BoardGameGeek