![]() ![]() If we generalise the rule above we get:ġ: only 2 candidates for a value, in each of 2 different units of the same kind, 2: and these candidates lie also on 2 other units of the same kind, then all other candidates for that value can be eliminated from the latter two units. We can also extend the idea to boxes as well. X-Wing is not restricted to rows and columns. If we had two pairs in two columns and those four numbers shared two rows, then we can eliminate any other occurrences of those numbers on the same rows. This strategy works in the other direction as well. This is good news because this leaves only a 9 at G9 and we can complete. We can remove the 6's marked in the cyan squares. If this is the case then any other 6's along the edge of our rectangle are redundant. Because A and CD are 'locked' then D must be a 6 if A is. If A turns out to be a 6 then it rules out a 6 at C as well as B. These have been highlighted with red boxes. What is interesting is the 6's present in the two columns 6 and 9 directly between A and C and B and D. Likewise if C turns out to be a 6 then D cannot be, and vice versa. We know therefore that if A turns out to be a 6 then B cannot be a 6, and vice versa. They are locked because they are the only 6's in the first and last rows. What's special about them? Well, A and B are a locked pair of 6's. The X is formed from the diagonal correspondence of squares marked A, B, C and D. The above picture shows a classic x-wing, this example being based on the number six. ![]() The reverse is also true for 2 columns with 2 common rows The rule isġ: only two possible cells for a value in each of two different rows, 2: and these candidates lie also in the same columns, then all other candidates for this value in the columns can be eliminated. ![]() It should be easier to spot in a game as we can concentrate on just one number at a time. This strategy is looking at single numbers in rows and columns. ![]()
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