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.)

Sudoku X-Wing example

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



What People Say

Well, I love SuDoku, and I collect SuDoku games—especially on my PalmOS-based Zire72... but I have several other WindowsXP-based SuDoku games too, and I feel that I must say about your game that I am impressed! Thanks for a well-written SuDoku program. (And, yes well, I really don't know why I collect different SuDoku programs; they each have their strengths and weaknesses, I guess, and I suppose in the future I will settle for using one in particular. But I find that I often turn to YOUR version, when I wish to just play a quick round, or explore the differences between the easier and the more difficult or expertly-rated puzzles. I appreciate ease-of-number entry when I want to concentrate on playing, rather than program mechanics!)
Mark Seven Smith, California
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