A Sudoku puzzle can be solved by applying the basic rules of Sudoku. These say that in a 9x9 Sudoku, each number from 1 to 9 can only appear once in each column, each row and each box. If a number has already been revealed or is given, it is excluded from all other cells in this row, column or box.

In X-Sudoku, this rule also applies to the two highlighted diagonals. These may also contain each number from 1 to 9 only once. This also creates dependencies between the boxes along the diagonal, which do not exist in classic Sudoku and can help the player solve the Sudoku.

In the example, the marked four for the box in the middle can also be used for **F4** can be excluded because it has already been revealed on this diagonal.