Sudoku Puzzles 129 - Associated Pairs, X-Wing

Solving methods > 4- Complex methods > 4.1 Associated pairs (X.wing)

The following methods are not necessarily more difficult to understand that the above ones, but it is not easy to spot when to apply them.

For a given number, when it’s only possible in two cells of two different rows, and that the four possibilities are on two columns (and form a rectangle), then that candidate can be excluded for the other cells of the column.
The same method applies in the case of two columns, the excluded candidates will then be on the rows.

For column B and H, the 9 is only possible on the 2nd and 8th row (blue cells). 9 can thus be eliminated from the other cells of these rows.

For column B, if 9 is not in B2, it is in B8 and inversely in column H. There will thus be necessarily a 9 in the 2nd and 8th rows, either in B2 and H8 or B8 and H2.

The 9 is thus eliminated from the rest of these rows

This method can be apply to groups :

As in the preceding example, there are two columns (B and C) where 9 can be in only two possible cells each (B3 and B9, C2 and C8).

Because B3 and C2 as well as B9 and C8 are in the same block (instead of the same row as previously), 9 can thus be excluded from the other cells of these two blocks.

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