Dots and Boxes

Close a square and it is yours — and you go again

Boxes left0

Boxes

Opponent

Strength

Board

Mark the giveawaysDots the lines that would put a third side on a box and hand it over.

How to play

The board starts as a grid of dots. A turn is one line joining two dots next to each other, across or down.
Close the fourth side of a square and the box is yours. It gets your letter, and you take another turn straight away — as many as you can keep closing.
The game ends when every line has been drawn. Most boxes wins, and an equal split is a draw.
Give a box a third side and your opponent can close it on their next turn, so most of the game is spent drawing lines that don't.
Sooner or later nothing safe is left, and whoever moves must open a run of boxes for the other — that is where the game is won and lost.
Boards run from 2×2, which takes a minute, to 5×5 for a proper game. Every board has an even number of boxes except 3×3 and 5×5, where a draw is impossible.
How the computer plays: it counts the chains, and from the endgame onwards it works the position out exactly, move by move, to the last line. It knows the double-cross — so it will hand you two boxes to keep you moving.
Online friend sends a link. You each play on your own device and both boards follow the same list of lines, so they cannot disagree.

Strategy

Count before you draw. A line is safe if neither square beside it already has two sides. Play those first and make your opponent run out of them before you do.
Chains. When the safe lines are gone, the empty boxes are joined into chains: open one and your opponent takes every box in it. The player forced to open the first long chain — three boxes or more — usually loses.
The double-cross. Handed a chain, don't take all of it. Take all but the last two, then draw the line through those two instead of closing them. Your opponent gets two boxes but has to open the next chain for you — and you take the whole of that one.
The same trick works on a loop: leave the last four boxes rather than two.
Sacrifice early to control the end. Handing over a box or two in the middle of the game can change whose turn it is when the chains open, which is worth far more than the boxes it cost.
Stuck? Hint plays the move the Hard computer would choose.