Dots and Boxes

Dots and Boxes (also known as Boxes, Squares, Paddocks, Square-it, Dots and Dashes, Dots, Smart Dots, Dot Boxing, or, simply, the Dot Game) is a pencil and paper game for two players (or sometimes, more than two) first published in 1889 by Édouard Lucas.

Game of dots and boxes on the 2×2 board.

Starting with an empty grid of dots, players take turns, adding a single horizontal or vertical line between two unjoined adjacent dots. A player who completes the fourth side of a 1×1 box earns one point and takes another turn. (The points are typically recorded by placing in the box an identifying mark of the player, such as an initial). The game ends when no more lines can be placed. The winner of the game is the player with the most points.

The board may be of any size. When short on time, 2×2 boxes (created by a square of 9 dots) is good for beginners, and 6×6 is good for experts. In games with an even number of boxes, it is conventional that if the game is tied then the win should be awarded to the second player (this offsets the advantage of going first).

The diagram on the right shows a game being played on the 2×2 board. The second player (B) plays the mirror image of the first player's move, hoping to divide the board into two pieces and tie the game. The first player (A) makes a sacrifice at move 7; B accepts the sacrifice, getting one box. However, B must now add another line, and connects the center dot to the center-right dot, causing the remaining boxes to be joined together in a chain as shown at the end of move 8. With A's next move, A gets them all, winning 3–1.

Strategy

The double-cross strategy. Faced with position 1, a novice player would create position 2 and lose. An experienced player would create position 3 and win.

At the start of a game, play is more or less random, the only strategy is to avoid adding the third side to any box. This continues until all the remaining (potential) boxes are joined together into chains – groups of one or more adjacent boxes in which any move gives all the boxes in the chain to the opponent. A novice player faced with a situation like position 1 in the diagram on the left, in which some boxes can be captured, takes all the boxes in the chain, resulting in position 2. But with their last move, they have to open the next (and larger) chain, and the novice loses the game,

An experienced player faced with position 1 instead plays the double-cross strategy, taking all but 2 of the boxes in the chain, leaving position 3. This leaves the last two boxes in the chain for their opponent, but then the opponent has to open the next chain. By moving to position 3, player A wins.

The double-cross strategy applies however many long chains there are. Take all but two of the boxes in each chain, but take all the boxes in the last chain. If the chains are long enough then the player will certainly win. Therefore, when played by experts, Dots and Boxes becomes a battle for control: An expert player tries to force their opponent to start the first long chain. Against a player who doesn't understand the concept of a sacrifice, the expert simply has to make the correct number of sacrifices to encourage the opponent to hand him the first chain long enough to ensure a win. If the other player also knows to offer sacrifices, the expert also has to manipulate the number of available sacrifices through earlier play.

There is never any reason not to accept a sacrifice, as if it is refused, the player who offered it can always take it without penalty. Thus, the impact of refusing a sacrifice need not be considered in your strategy.

Experienced players can avoid the chaining phenomenon by making early moves to split the board. A board split into 4x4 squares is ideal. Dividing limits the size of chains- in the case of 4x4 squares, the longest possible chain is four, filling the larger square. A board with an even number of spaces will end in a draw (as the number of 4x4 squares will be equal for each player); an odd numbered board will lead to the winner winning by one square (the 4x4 squares and 2x1 half-squares will fall evenly, with one box not incorporated into the pattern falling to the winner).

A common alternate ruleset is to require all available boxes be claimed on your turn. This eliminates the double cross strategy, forcing even the experienced player to take all the boxes, and give his opponent the win.

In combinatorial game theory dots and boxes is very close to being an impartial game and many positions can be analyzed using Sprague–Grundy theory.

Unusual grids

Dots and boxes need not be played on a rectangular grid. It can be played on a triangular grid or a hexagonal grid. There is also a variant in Bolivia when it is played in a Chakana or Inca Cross grid, which adds more complications to the game.

Dots-and-boxes has a dual form called "strings-and-coins". This game is played on a network of coins (vertices) joined by strings (edges). Players take turns to cut a string. When a cut leaves a coin with no strings, the player pockets the coin and takes another turn. The winner is the player who pockets the most coins. Strings-and-coins can be played on an arbitrary graph. A variant played in Poland allows a player to claim a region of several squares as soon as its boundary is completed.

References

• http://cgi.cae.wisc.edu/~dwilson/play.php?moves=&opt=NB interactive dots and boxes games
• Elwyn Berlekamp (July 2000). The Dots-and-Boxes Game: Sophisticated Child's Play. AK Peters, Ltd. ISBN 1-56881-129-2. 
• Barile, Margherita, "Dots and Boxes" from MathWorld.
• David Wilson, Dots-and-Boxes Analysis. Contains computer analysis of small boards.
• Ilan Vardi, Dots Strategies.
• A paper and pencil game that combines Dots and Boxes with Chess, Connect Score .