Duck chess

Duck chess, created by Tim Paulden in 2016, adds a neutral object to the board, usually represented by a rubber duck. The duck belongs to neither side. Both players control it on their turns and use it to close or reopen lines.

The two actions in every turn

1. Make a move with one of your own pieces, following its usual movement and this variant's exceptions.
2. Then place the duck on any empty square. If it is already on the board, it must change squares rather than remain in place.

The duck is a blocker. No piece may occupy its square, and sliding pieces such as rooks, bishops, and queens cannot pass through it. A knight may jump over it because the knight does not travel through intervening squares. The duck cannot be captured.

Check, which in ordinary chess means that the king is attacked, and checkmate, an attack with no legal reply, do not exist. A king may remain under attack or move onto an attacked square, and victory comes from physically capturing the opposing king. A player may therefore expose the king with the piece move and then use the duck to block the capture, although protecting the king is not compulsory.

Castling, the joint king-and-rook move, is commonly retained when both pieces still have the right and the path is clear. Because check does not exist, restrictions involving attacked squares normally do not apply. Some published rules also award victory to a player with no possible move, a situation called fowling; the platform's implementation should be checked.

Every turn changes the position twice. Good calculation therefore asks not only what the piece move accomplishes, but also which attack the duck will block, which defender it will isolate, and which line it will uncover when it leaves its old square.