Zugzwang is a concept which is frequently seen in chess, but also exists in other games. It literally means "forced move" and it is defined as follows:

> a situation in which the obligation to make a move in one's turn is a serious, often decisive, disadvantage.

Zugzwang in Chess

One of the most simple examples of it occurring in a chess game is in a king vs king + pawn ending. Here, with black to move, zugzwang is used to force the enemy king away from the promotion square.

In this position if it is black's move they would prefer if they could just pass, then white has no way to win. But since in chess you must make a move they must play 1...Kf7. Then white can play 2.Kd7 and 3.e8=Q on the next move, winning.

If it is white to move first then 1.Ke6 is stalemate which is a draw.

In chess it is nearly always an advantage to be the player to move, but in zugzwang positions like the position above it is a disadvantage.

Zugzwang in Other Board Games

Zugzwang also occurs in many other games, but not every game. For example in Tic Tac Toe it would never be an advantage to pass because it would just let your opponent get closer to winning without hindering them in any way.

Some games like Quarto are centred around the concept of zugzwang. The only way to win is if your opponent selects a winning piece and gives it to you. Unless they make a careless mistake, they will only do this if it is zugzwang and all their possible moves would lose.

Zugzwang in Battlesheep

Battlesheep is a fun family-friendly game which is easy to learn, but has plenty of strategy despite simple rules. And it also surprisingly can feature zugzwang in some situations.

The idea of the game is to control as many tiles as possible with your sheep while prevent your opponent(s) from doing the same. Each tile is worth one point. If the score is tied at the end of the game, then the player with the largest connected group of tiles wins.

A move consists of choosing one stack of your own sheep and splitting it into two stacks. One stack (at least one sheep) must remain where it is, and the other stack moves in a straight line in any direction until it hits the edge of the board or another stack of sheep. You can use your sheep to block off sections of the board to prevent your opponents getting in.

The first time you play it seems like it would always be an advantage to be the player to move. But even with these simple rules it is possible to create situations where being forced to move can cause you to lose!

In the above position both players have claimed 6 tiles. The red player has only one valid move and the blue player has two moves to choose from.

In this position, whichever player has the turn, they will lose!

If blue moves first, red wins

If blue goes first they are forced to move one of their sheep so that it blocks their other stack, then red wins with the tie-break rule. The following photo shows the position after blue moves one sheep from their rightmost stack to the left, blocking their left stack of sheep.

If red moves first, blue wins

If red goes first they must play their only move: to place a sheep in the centre square. Then when blue moves they no longer are forced to block their own sheep so they can win with 8 tiles to red's 7.