While not as famous as Gary Gygax, Dave Arneson is the person who is arguably the one most responsible for the nuts and bolts basics of Dungeons & Dragons. Arneson met Gygax at the 1970 Gen Con, where the two bonded over a shared love of wargames and the fact that both were members of the International Federation of Wargamers. The two first collaborated on a naval wargame called Don't Give up the Ship that was published by Gygax's Guidon Games in 1971. It wasn't until the 1971 Gen Con, however, that the collaboration between them began that would ultimately culminate in Dungeons & Dragons.

At that show, Arneson had created a miniature scenario using a variant of Gygax's Chainmail rules that involved a commando team of soldiers sneaking into a castle to open a drawbridge. The scenario proved so popular that Arneson' gaming group began creating more like it, eventually turning a generic castle map into "Blackmoor," one of D&D's original campaign settings. It would be Arneson's Blackmoor campaign that first tossed out the "either/or" combat matrix of the original game, adding in innovations such as "hit points" to determine how wounded a character was and the idea of advancing levels and experience points to indicate growing power. He also moved the game away from set-piece battles and castles into underground spaces filled with monsters and treasures -- the first "dungeon crawls."