zebedee wrote:Confusion reigns - at least in my mind.

X-wings tend to unfortunately have that effect. I was tearing my hair out over the first puzzle I encountered with an x-wing.

zebedee wrote:Are you saying scrose that the X-Wing comprises c1/r2, c8/r2, c1/r9 and c8/r9?

Yes, that's correct.

zebedee wrote:If so, I don't see how this can be an X-wing since, according to my pencil marks, 9s can still go in c7/r2 and c7/r9. It would also seem that a 9 can go in c3/r9 as well.

Those candidate 9's in r2c7 and r9c7 are the ones that you can eliminate due to the candidate 9's in r2c1, r2c8, r9c1, and r9c8.

For a moment, focus only on the candidate 9's in rows 2 and 9.

- Code: Select all
` . . . | . . . | . . .`

9 . . | . . . | 9 9 9

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-------+-------+-------

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-------+-------+-------

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9 . 9 | . . . | 9 9 .

Suppose we place a 9 at r2c1. That would eliminate the remaining candidate 9's in row 2 in block 3, and the candidate 9 at r9c1.

- Code: Select all
` . . . | . . . | . . .`

9 . . | . . . | . . .

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-------+-------+-------

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-------+-------+-------

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. . 9 | . . . | 9 9 .

Note this would leave the candidate 9 at r9c8 as the only candidate 9 in column 8. Thus the other candidate 9's in row 9 can be eliminated.

- Code: Select all
` . . . | . . . | . . .`

9 . . | . . . | . . .

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-------+-------+-------

. . . | . . . | . . .

. . . | . . . | . . .

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-------+-------+-------

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. . . | . . . | . 9 .

So, that is one diagonal wing of the x-wing. Let's look at the other possibility.

- Code: Select all
` . . . | . . . | . . .`

9 . . | . . . | 9 9 9

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-------+-------+-------

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. . . | . . . | . . .

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-------+-------+-------

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9 . 9 | . . . | 9 9 .

Suppose we place a 9 at r2c8. That would eliminate the remaining candidate 9's in row 2, and the candidate 9 at r9c8.

- Code: Select all
` . . . | . . . | . . .`

. . . | . . . | . 9 .

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-------+-------+-------

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. . . | . . . | . . .

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-------+-------+-------

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. . . | . . . | . . .

9 . 9 | . . . | 9 . .

Note this would leave the candidate 9 at r9c1 as the only candidate 9 in column 1. Thus the other candidate 9's in row 9 can be eliminated.

- Code: Select all
` . . . | . . . | . . .`

. . . | . . . | . 9 .

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-------+-------+-------

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. . . | . . . | . . .

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-------+-------+-------

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9 . . | . . . | . . .

This is the other diagonal wing of the x-wing.

That means the only place 9 can be in row 2 is either in column 1 or 9 and the only place 9 can be in row 9 is either in column 1 or 9.

I hope this helped!