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X Wings |
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An X-Wing is where two columns have a candidate in only two different rows, or vice versa. This will always involve 4 cells in a square arrangement, often considered as endpoints of an X arrangement. The columns or rows that have these candidates are the primary units, and must only contain the candidates in the other two rows or columns called the secondary units. All other candidates in the secondary units can be eliminated.
Consider the example on this page. Columns A and E only have 4's in rows 2 and 8. Thus, all other 4's in rows 2 and 8 can be eliminated.
Sudoku Snake gives X-Wings a skill value of 45.
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