X-Wing

An X-Wing occurs when in two rows all possible cells for a given digit lie in two columns. This digit can be eliminated from the remaining cells of these columns. The technique also works when we replace rows with columns.

In the following sudoku diagram in the third and seventh rows, the digit 5 can only be placed in the first and eighth columns (candidates marked in blue). There are only two options for placing the digit 5 in the third and seventh rows. In both cases, both the first and the eighth columns will contain the digit 5. Therefore, the digit 5 (marked in red) can be eliminated from the remaining cells of these columns. After using this technique, the digit 5 in the right middle box can only be placed in the R4C7 cell.