Checkers

Checkers — English draughts outside North America — is played on the dark squares of an 8×8 board with twelve men a side. Red moves first. Men move one square diagonally forward and capture by jumping an enemy piece to the empty square directly beyond it. A man who reaches the far row is crowned a king, and a king moves and captures in all four diagonal directions. You win by taking all your opponent's pieces, or by leaving them with no legal move at all.

Tap one of your pieces to pick it up: the squares it may move to are marked, and tapping one plays the move. A jump that can carry on will only offer you its next hop, so a double or triple jump is made one square at a time and counts as a single move. Play two-up on one device, or against the computer at three strengths.

Captures are compulsory

This is the rule that catches people out, and it is the real rule: if you can capture, you must. The board here will refuse a quiet move while a jump is on offer, and tells you so above the board. Where two or more captures are available you may choose freely between them — you are not obliged to take the longest chain. That "majority capture" rule belongs to international draughts, a different game on a 10×10 board, and is not played here.

Compulsory capture is what makes checkers a game of shots rather than a game of shuffling. Because your opponent has to take what you offer, a piece given away on purpose can drag their piece onto a square where you take two back. Every strong player's tactics start here.

Kings, and the one rule about crowning

A man is crowned the moment he reaches the opposite back row, and crowning ends the move. If a jumping man lands on the crown row with another jump still available, he is crowned and stops there; he does not carry on as a king. That single rule decides a surprising number of endings, and it is the one most casual implementations get wrong.

Kings here are short kings: they move and capture one square at a time in any diagonal direction, as English draughts has it. The flying kings that slide any distance along a diagonal are, again, international draughts.

Losing without losing every piece

A player who cannot move has lost, even with pieces still on the board. Blocking your opponent's last mobile piece is a perfectly good way to win, and it is why the endgame is about mobility rather than material: two kings that have cornered a man have won the game whether or not they ever take it.

Games that neither side can win end as draws. This board declares one after fifty moves in which nothing has been captured and no man has moved — the point at which the position can no longer make progress, since a man can never move backwards.

How to play well

Keep your back row intact early. Those four men cannot be crowned through, so an opponent who has broken up his own back row has given you a road to a king; the computer here counts that in its judgement of a position.

Push your men forward as a connected front rather than as individuals. A man with a friendly piece diagonally behind him cannot be taken for free — the jumper lands where you can take it straight back. Most beginners' losses are single men wandered forward alone.

Then count before you take. Because captures are forced, the question is never "can I win a piece" but "what does taking it force me into next". A two-for-one shot looks like a gift on the first move and a lost game on the third.

The three computer levels

Easy plays at random among its legal moves. It still takes captures — it has to, they are compulsory — but it will hand you pieces for nothing. Medium searches four plies, two moves for each player: it will not give a piece away and it takes what you leave hanging, but it does not see the shots that set up a two-for-one. Hard searches seven plies with alpha-beta pruning, judging the positions it reaches by material, how far the men have advanced, whether the back row is intact, and who holds the centre. It plays the shots, and it punishes a loose man several moves before the consequence is visible.

None of the three is perfect play. Checkers was solved in 2007 by the Chinook team at the University of Alberta, after eighteen years of computation: with best play from both sides the game is a draw. A perfect opponent is a database of endgames rather than a search, which is far too large to send to a browser — so Hard is a strong practical player and can be beaten.