Tic-tac-toe was a Human game in which two players tried to fill three boxes in a row within a 3×3 square grid with either an X or an O (vertically, diagonally, or horizontally).
In the 2260s, a three-dimensional version of the game was often played in the recreation room aboard the USS Enterprise. (TOS: "The Naked Time", "Charlie X", "The Conscience of the King", "The Alternative Factor", "The Trouble with Tribbles", "By Any Other Name")
In 2373, when Harry Kim referred to kal-toh as "Vulcan chess", Tuvok commented that "Kal-toh is to chess as chess is to tic tac toe." (VOY: "Alter Ego")
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