How to use the checkers solver
The checkers solver starts from the normal opening position with Black at the top; Black moves first. To analyze a game, play the moves on the board: tap a piece, then the square it moves to. For a jump, tap the square where the piece finally lands, even after a double or triple jump. To copy a position from a book or a real board, choose Set up, pick a piece (black, black king, white, white king or empty) and tap the dark squares. Then set the side to move and press Find the best move.
The small numbers on the dark squares are the standard notation: 1 to 32, starting from Black's side. Moves are written 11-15 for a step and 15x24 or 15x24x31 for jumps, with every landing square. You can also paste a position as a checkers FEN, such as B:W18,24,27,K10:B12,16,K22.
The rules it follows
- Pieces move one square diagonally forward onto a dark square; kings move one square in any diagonal direction.
- Captures are compulsory. If any jump is available you must take one, and you must keep jumping with the same piece while you can. You may choose which jump to make when there are several, it does not have to be the longest.
- Men capture forward only; kings capture both ways.
- A man that reaches the far row is crowned and the move ends there, even if the new king could jump again.
- You win when the other side has no piece left or cannot move.
The solver checks your position before searching: no more than 12 pieces a side, and no uncrowned man on the opponent's back row (it would already have been crowned).
How the solver works
The engine is our own. It looks ahead with alpha-beta search, the classic game-tree method that skips lines which cannot change the result, and it deepens one move at a time until the time you chose runs out (iterative deepening). A transposition table remembers positions it has already reached by another move order, and jump sequences are always searched to the end so a capture never hides behind the horizon. Positions are scored on material (a king is worth about one and a half men), advancement, the back-row guard and center control.
When the search sees a win that the other side cannot avoid, the result says so: "Black wins, forced win in 7 Black moves". Otherwise the evaluation is an estimate, in men. We test the engine against a separate brute-force search that tries every reply: on hundreds of positions, every forced win it reported was confirmed, with no faster win missed. Move generation is checked against the published counts of all possible games from the start position, up to 9 moves deep (3,963,680 positions).
Worked example
Press Example: Black has kings on 1 and 6 and a man on 8; White has a king on 3 and men on 7 and 25. Black to move wins by force with 6-2. White must jump 3x12 (captures are compulsory), and Black answers 2x11, taking the man on 7. That leaves two black kings against a white king and a man, and Black wins on the 7th move whatever White does; the solver shows the whole line.
Play against the solver
Choose Play it, your side and a level. Easy looks one move ahead and often picks a weaker move on purpose, Medium looks four moves ahead (two each), and Hard is the full solver with about 1.5 seconds of thought. In our test games, with the first two moves random and the colors swapped each game, hard beat medium in 7 of 8 games (one draw), medium beat easy in 9 of 10 (one draw) and easy beat random moves in 10 of 10. Hard also converts won king endings: from random positions it won 10 of 10 games with two kings against one, and 6 of 6 with three kings against two. A position that comes up for the third time is a draw. Undo takes back your move and its reply.
For another two-player board game against the computer, try nine men's morris; for chess positions, the chess move calculator.
Solver tested . Found a problem? Tell us.
Questions
Which checkers rules does the solver use?
American checkers, also called English draughts: an 8x8 board, 12 pieces each, Black moves first, compulsory captures, men capture forward only, kings move one square at a time, and crowning ends the move. International draughts (10x10, flying kings) is not supported.
Do I have to take the longest jump?
No. In American checkers you must capture if you can, and you must finish the jump sequence you start, but you may choose any of the available captures, not only the one that takes the most pieces.
Is checkers solved?
Yes. In 2007 Jonathan Schaeffer's team proved that checkers from the standard opening is a draw with perfect play by both sides. This solver does not use their database; it searches each position in your browser, so in the opening it gives a strong move, not a proof.
What does "forced win in 5 moves" mean?
Whatever the losing side plays, the winning side can take all the opposing pieces or leave them with no legal move within 5 of its own moves. The line shown is the defense that lasts longest.
Why does it say a capture is compulsory when I tap a piece?
In checkers you must jump when a jump is available. If any of your pieces can capture, only capturing moves are allowed, so pieces that cannot jump cannot move this turn.