This strategy helps to eliminate a candidate from cells too. It can then be eliminated as a candidate to that cell. Testing the pairs on the grid shows that the 5 highlighted in red would be impossible in both cases.

By connecting the cells containing it, it is easy to see that in this situation only the pairs in green or yellow could be possible. In this example, the number 5 forms the necessary pattern to apply the X-Wing strategy. The next step will be to test those sets on the grid and eliminate the digit from any cell that would become impossible in both situations. By making an X linking diagonally the two opposite extremities of this rectangle, the player finds only two possible sets of positions for that digit.

The player can use this strategy when there is one candidate repeated in four cells that form a square or rectangle when mentally connected by row and column. It can also be applied in some intermediate levels, although its incidence is very low in these cases. The X-Wing method is one of the most basic advanced Sudoku strategies. Regardless, their application always demands high levels of concentration from the player as they work by deduction. Advanced Sudoku strategies are used in the hardest levels of these puzzles and they can either help to reduce candidates or to find the solution for a specific cell.