# Solving Sudoku - X-Wing

In the Sudoku puzzle below, the only cells in rows one and eight that can contain a 9 are those coloured green.  Since there must be a 9 in both row one and row eight, but they cannot occupy the same column, it follows that either the top-left and bottom-right marked cells contain the 9s, or the bottom-left and top-right cells do. (It can't be the bottom-right and top-right, nor the bottom-left and top-left, as then there would be two 9s in the same column. Similarly, it can't be top-left and top-right, nor bottom-left and bottom-right as then there would be two 9s in the same row.) So, we can't say whether the 9s are in top-left and bottom-right, or bottom-left and top-right, but either way, it excludes 9s from the other cells in both columns. The end result is that 9 can be eliminated from the candidates for other cells in both of the affected columns (coloured blue in this example.)

Here are some Sudoku puzzles that can be solved using the X-Wing technique: (What are .sdk files?)

What People Say

It's a wonderful Sudoku program! I like it because I deliver Sudoku puzzles to newspaper in Copenhagen and North Zealand and it does a lot of work for me. It's just wonderful. Instead of calling it sadman.... you should call it happyman....
Douglas Fullerton, Denmark.
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