What is Chess Sudoku

Chess sudoku refers to a range of different sudoku variants that incorporate elements of chess, usually in terms of additional constraints but sometimes also in terms of the shape and size of the grid. 

In most variants, normal sudoku rules apply (each 3×3 block, 9 cell row, and 9 cell column must contain the numbers 1 through 9 only once). Additional constraints are then added to where numbers can appear based on the moves of various chess pieces. 

These moves of various chess pieces result in different sub-variants of chess sudoku, including: 

Knight’s move sudoku

Knight’s move sudoku is one of the more popular forms of chess sudoku. 

In addition to the regular sudoku rules, in this variant the same number cannot appear within a ‘knight’s move’ of itself. That is, the same number cannot appear two cells vertically and one cell horizontally of another instance of itself and it cannot appear one cell vertically and two cells horizontally of another instance of itself. 

This path forms an ‘L’ shape as shown below. 

When you take into account all the other possible cells within a ‘knight’s move’ of the center cell, it means that in a knight’s move chess sudoku, whatever number appears in the blue center cell below, cannot appear in any of the yellow cells. 

As normal sudoku rules still apply, whatever number appears in the center blue cell cannot appear within the same 3×3 square or in any of the same cells in its column or row. 

These cells that are eliminated by regular sudoku rules are highlighted in yellow below in addition to the cells eliminated by the knight’s move constraint. 

King’s move sudoku

Another popular type of chess sudoku is when a ‘king’s move’ constraint is added. 

In chess, a king is able to move to any neighboring square (including diagonally). Therefore, in the king’s move variant of chess sudoku, the same number cannot appear one cell away for another instance of itself even if it is in a different column, row, or 3×3 square. 

In the diagram below, this means that whatever number appears in the blue cell cannot appear again in any of the yellow cells.

Again, normal sudoku rules and constraints apply. All the cells that whatever number in the blue cell cannot appear again in, as per the king’s move constraint and regular sudoku rules, are highlighted in yellow below.