Chess was a game of strategy that had its origins on Earth. While originally conceived as a two-dimensional game, by the 23rd Century, a three-dimensional variant of the game had become popular. James T. Kirk and Spock frequently played chess together, and the Vulcan was often surprised by his friend's illogical—but effective—strategies. "In chess," Spock once observed, "when one is outmatched, the game is over. Checkmate."