Checkers

Strategy

The rules, precisely

Checkers is played on the 32 dark squares of an 8×8 board, set so that each player has a dark square in the corner nearest their left hand. Each side starts with 12 men on the three rows closest to them. Black — the dark pieces — moves first, and the players then alternate. A man moves one square diagonally forwards onto an empty square. It captures by jumping diagonally over an adjacent enemy piece onto the empty square directly beyond, and the jumped piece is removed. Capturing is compulsory: if you can take, you must, although when several captures are available you choose which. A piece that lands where it can jump again has to keep jumping in the same turn. A man that reaches the far row is crowned a king, and in American checkers that ends the move even if the new king could jump again. Kings move and capture one square diagonally in any direction. You win when your opponent has no pieces left, or no legal move on their turn.

Three rule sets on one board

Choose the rules in the panel beside the board. American checkers — the same game the British call English draughts — is described above and is the default. International draughts is played on a 10×10 board with 20 men each and White moving first. Men still move forwards but capture both forwards and backwards; kings fly, travelling any distance along an open diagonal and taking a piece from far away, landing on any empty square beyond it; and you must choose the capture that takes the most pieces. A man that touches the far row in the middle of a capture and then jumps on is not crowned. Russian draughts keeps the 8×8 board and 12 men, with White first, backward captures and flying kings, but lets you pick any capture sequence — and a man that reaches the far row during a capture is crowned on the spot and carries on jumping as a king. In the flying-king games, captured pieces stay on the board until the move is over and can never be jumped twice, the so-called Turkish strike rule. All of it is enforced here, so the board only ever offers the moves your chosen rules allow.

Compulsory captures change everything

Because a capture must be taken, you can force your opponent's hand. The classic tactic is the shot: you deliberately give up a piece so that the forced recapture drags an enemy man onto a square where you take two in return. Strong players read every move in terms of what it obliges the other side to do next. Two habits follow from that. Before you move, check whether your move offers a capture, and where the capturing piece would land. And when you have a choice of captures, prefer the one that leaves your pieces protected over the one that simply looks biggest.

Keep your back row — for a while

A man on your back row stops the opponent from crowning on that square, and kings decide most endgames. American players talk about the bridge: men on squares 1 and 3 for Black, or 30 and 32 for White, which together make it very hard for a single enemy man to break through and crown. You cannot keep them there forever — sooner or later they are needed in the fight — but moving them early for no reason is one of the most common ways to hand your opponent a king.

Take the centre

The four centre squares — 14, 15, 18 and 19 in the standard numbering — are where a piece has the most options and does the most blocking. A piece on the side of the board cannot be captured, but it can only move one way, and a side that has been pushed to the edges runs out of useful moves. The most popular opening move in American checkers, 11-15, heads straight for the centre.

Trade when you are ahead

One extra piece means far more in an endgame of three against two than at twelve against eleven. When you are ahead, exchange pieces one for one; when you are behind, avoid trades and keep the position tangled. The computer here is built to do exactly that, which is why it can seem to get stronger as the board empties.

Kings and the double corner

In American checkers a lone king can shelter in a double corner — the two dark squares that meet in one corner of the board — where a single enemy king can never trap it, so one king against one king is a draw. Two kings against one is a win, but only once you have driven the defender out of the double corner, which takes a little technique. In the flying-king games a king is worth several men, and the long diagonal running from corner to corner, known as the main road, is the most important line on the board.

Is checkers solved?

Yes. In 2007 a team led by Jonathan Schaeffer at the University of Alberta published a proof that American checkers is a draw with perfect play from both sides. The computation had been running, on and off, since 1989, working through a game with about 500 billion billion positions. Their program, Chinook, had already made history in 1994 as the first computer program to win a world championship title against human players. International draughts, with its bigger board, has not been solved.

What the difficulty levels actually do

Nothing is faked: every level is the same alpha-beta search over the real rules, stopped at a different depth. Easy looks two moves ahead and plays a random legal move about three times in ten, so it gives pieces away while you learn. Medium looks four moves ahead and occasionally slips. Hard looks up to six moves ahead. Expert keeps searching deeper for as long as it has time, a little under a second per move. At every level a position with a capture pending is never treated as finished — the search plays the exchange out before judging it, which is what stops it walking into a two-for-one. Once it is ahead it is rewarded for trading pieces, and in a thin endgame for bringing its kings towards yours. The search is time-capped, so on a slow phone it plays a slightly shallower move rather than freezing the page.

Two players on one device

Switch the opponent to 2 players and the board passes back and forth on the same screen, with the side that moves first at the bottom. It is the mode for a café table, a classroom or a long train journey. Undo and the move list work exactly as they do against the computer, and results go into a separate session tally, so they never mix with your score against the computer.

Playing without a mouse

The arrow keys move between the dark squares, Enter or Space picks a piece up and puts it down, Esc cancels a half-entered move, U undoes, H shows a hint and N starts a new game. Every square is a real button with a label — its number, the piece on it and whether that piece can move — and the turn line is a live region, so the game can be followed by ear as well as by eye.

Nothing here leaves your device

The game runs entirely in your browser. There is no account and no server keeping your games, and the computer's search runs on your own device, which is also why it answers instantly. Your score, your settings and the game in progress are kept in your browser's local storage; clearing your browser data resets them, and nothing is stored anywhere else. If you enjoy the long-range planning, try Reversi for a game where the lead changes hands on every move, or Connect Four for something quicker.