What is Sudoku X?
Sudoku X, also called diagonal sudoku, is classic sudoku with one extra rule: the two long diagonals of the grid also contain the digits 1 to 9 exactly once. The diagonals are shaded on the grid, so you always see which cells they cover.
The rules
Every row, every column and every 3×3 box contains the digits 1 to 9 exactly once, as in any sudoku. In addition, each of the two diagonals, from the top-left corner to the bottom-right and from the top-right corner to the bottom-left, contains the digits 1 to 9 exactly once. The two diagonals form the X that gives the puzzle its name.
The two shaded diagonals each hold 1 to 9 once. The centre cell is on both.
How to get started
- The centre cell belongs to a row, a column, a box and both diagonals. It is the most constrained cell on the grid, so check it early.
- Whenever you place a digit on a shaded cell, remove that digit from every other cell of the same diagonal. This is the step most people forget at first.
- Each diagonal passes through three boxes, three cells in each. The three diagonal cells of a box cannot share a digit with the six diagonal cells in the other two boxes.
- Sudoku X puzzles start with fewer given digits than a classic sudoku. The diagonals do part of the work, so use them from the first step, not only when you are stuck.