Winning by blockade: when having no move loses

A player with no legal move loses, even with pieces left on the board. Example blockades on both boards, what to look for, and how to avoid being shut in.

You do not have to capture everything to win. In all four rule sets on this site, a side to move that has no legal move loses at once, however many pieces it still has. The game screen gives the reason as "No move left", followed by the name of the side that is stuck.

What blocked means

A man needs one of two things to move: an empty square to step to, or an enemy piece with an empty square behind it to jump. If no piece of a player has either when their turn comes, that player has lost.

American checkers, White to move: the man on 32 cannot step to 27 or 28, cannot jump 27 because 23 is occupied, and cannot jump 28 because no square lies behind it on the edge, so White has no legal move and Black wins.

Only three of those black men do the work: 27 and 23 close one line, 28 closes the other by itself, and 24 is a spare. In a corner it takes only two.

International draughts, White to move: the man on 46 has one forward square, 41, and the jump over it needs 37, so two black men win the game.

More pieces do not save you

American checkers, White to move: three white men against two black men, and White has no legal move, so Black wins.

The men on 5 and 6 look one step from crowning. But they can only step to 1 and 2, which Black occupies, and there is no square beyond the king row to jump to. The man on 9 is stuck behind its own men. The game ends here without another capture.

A man racing for the king row needs an empty square when it gets there, so count the men on the back row before you send everything forward.

Closing the door yourself

American checkers, White to move with three legal moves: the engine's first choice, searching six plies ahead, is 13-9.

Black's man on 5 stands on the side edge, so it has only one forward square, 9. The move 13-9 occupies it, and the jump over 9 needs 14, which White's other man holds.

Black to move after 13-9: the man on 5 has no step and no jump, so the game is over and White wins with both men on the board.

Compare 14-9. It occupies the same square but empties 14, so Black now has a capture and must take it. A blockade needs the step and the jump closed at the same time.

What to look for

Edge squares. A man on a side edge, like the one on 5 above, has one forward square instead of two, so it takes fewer pieces to stop.
Two in a row. Two pieces next to each other on a diagonal close both the step and the jump along that line, as 27 and 23 do in the first position.
Spare moves. Once the men have locked together, count how many safe moves each side has left. Whoever runs out first must give up a man or, like Black after 13-9, has no move at all.

How to avoid being shut in

Keep one piece free. A blockade has to stop every piece you own. Give White one more man with a free square in the first position and the game goes on, and a king with an open diagonal is harder still to shut in.
Look before you advance. Before a man steps onto the edge or up against the king row, check that it will still have a move on your next turn. The black man on 5 had a single forward square, and one white move took it away.
Break the wall with an offer. Capture is compulsory for the side blocking you as well. Offer a man to a piece in the wall and, if that is the only capture, it has to take and leave its square. Forcing captures shows how to choose the offer.

Next time you play a computer opponent on the play page, count each side's free moves once the board thins out. You may find a win that needs no capture at all.