Kal-toh is a Vulcan game of intense strategy that can be played solitary or with multiple individuals. The object of kal-toh, as stated by Tuvok, "Is not about striving for balance but about finding the seeds of order even in the midst of profound chaos."
When Harry Kim called kal-toh "Vulcan chess", Tuvok commented that, "Kal-toh is to chess as chess is to tic tac toe." (VOY: "Alter Ego")
In a holodeck simulation created by the Doctor that included numerous historical figures, the holograms of Socrates and T'Pau played kal-toh together. (VOY: "Darkling")
During a game played by Tuvok and Harry Kim, Seven of Nine took a turn for Harry and beat Tuvok in one move, claiming she used elementary spatial harmonics to do so. (VOY: "The Omega Directive")
Tuvok, Icheb and Harry Kim often had a match of kal-toh together. Icheb was quite talented with the game. (VOY: "Endgame")
Trivia
The external portion of a completed game of kal-toh is an icosidodecahedron.