Teeko is an abstract strategy game invented by John Scarne in 1937 and rereleased in refined form in 1952 and again in the 1960s. Teeko was marketed by Scarne's company, John Scarne Games Inc.; its quirky name, he said, borrowed letters from Tic-tac-toe, Chess, Checkers, and Bingo.^[1]

The Teeko board consists of twenty-five spaces arranged in a five-by-five grid. There are eight markers in a Teeko game, four black and four red. One player, "Black" plays the black markers, and the other, "Red", plays the red.^[2]: 513

1. Black moves first and places one marker on any space on the board.
2. Red then places a marker on any unoccupied space.
3. Players continue to take turns placing markers on empty spaces until all eight markers are on the board.
4. Once all eight pieces are on the board, if neither player has won after this initial "drop" placement phase, then they move their pieces one at a time, with Black playing first.
5. A piece may be moved only to an adjacent space.

The object of the game is for either player to win by having all four of their markers in a straight line (vertical, horizontal, or diagonal) or on a square of four adjacent spaces. (Adjacency is horizontal, vertical, or diagonal, but does not wrap around the edges of the board.)

The rules, as summarized above, are very simple, but the strategy is complicated enough to fill a book, Scarne on Teeko (1955).^[3] Nonetheless, Guy L. Steele Jr. solved the game (i.e., showed what must occur if both players play perfectly) via computer in 1998 and found neither player can force a win.^[4] Steele also showed that the Advanced Teeko variant is a win for Black (again, assuming perfect play), as is one other variant, but the other fourteen variants are draws.^[4]

There are sixteen variations of Teeko, such as Advanced Teeko, which have slightly different rules. All sixteen are outlined in Scarne on Teeko; the rules above are for "Standard Teeko" (or "Teeko"). Steele showed that Advanced Teeko is a win for Black (assuming, again, that both players play perfectly), as is one other variation, but the other fourteen are draws.^[3]

• Connect Four
• Connect6
• Seega

• Browser-based version of the game
• Scarne, John (1973). "25: Chess, Checkers, and Teeko". Scarne's encyclopedia of games. Harper & Row. pp. 498–521. LCCN 72-79691. SBN 06-013813-0.