How to Solve Sudoku: X-Wing

In 2 rows, the places for a digit are only in the same 2 columns. Then the digit can be removed from the other cells of those 2 columns.

On Puzzle Lounge it appears from Oni up.

See it step by step

  1. A board full of notes. Look only at the 8s (the small digits are the notes, or candidates)
  2. ● = a cell where 8 can go (notes other than 8 have been removed)
  3. In row 5, 8 has only 2 places
  4. Row 9 too has only 2 places, in the same columns (the 4 ● form a rectangle)
  5. 8 can only go in these 2 ways (one diagonal or the other)
  6. Either way, the 8s of columns 2 and 9 are inside this rectangle
  7. So the other 8s can be removed (8 goes from row 8, column 2)

This is Oni No.2, partly solved.

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How to find it

  1. Pick a digit and find a row (or column) where it has only 2 candidates.
  2. See whether another row (or column) has its candidates in the same 2 columns (or rows).
  3. If you find one, remove the digit from the other cells of those 2 columns (2 rows).

Things to watch

  • In both of the 2 rows, the candidates must be exactly those 2 places.
  • The trick is to search digit by digit. Go through 1 to 9 in order.
Read the full explanation

When a digit's candidates in each of 2 rows lie only in the same 2 columns, the digit can be removed from the other rows of those 2 columns (and the same with rows and columns swapped).

Suppose a digit (say 8) can go only in columns X and Y in row A, and only in columns X and Y in row B. The 4 cells form a rectangle.

If row A's 8 is in column X, row B's 8 is in column Y. If row A's is in column Y, row B's is in column X. Either way, the 8s of columns X and Y are used up in rows A and B.

So 8 cannot go in the other rows of columns X and Y.

In rows 5 and 9, the candidates for 8 lie only in columns 2 and 9. So the 8s of columns 2 and 9 are used up in these 2 rows, and 8 can be removed from the candidates at row 8, column 2.

Other techniques

Hidden SingleNaked SinglePointingClaimingHidden PairHidden TripleCage Combinations (Killer)Rule of 45 (Killer)Naked PairNaked TripleXY-WingSwordfishSkyscraper2-String KiteXYZ-WingW-WingJellyfish

For the basics and how to use notes, see how to solve sudoku, with tips. If you get stuck, the game's hints guide you step by step, starting with where to look.

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