The digit is restricted to 2 or 3 cells in a row. To find the necessary pattern in a regular game, the player must ensure that: This basic configuration is the least recurrent on puzzles. The solution for the digit in each of the highlighted rows ensures that it cannot be present in any other position within the columns shared by the cells, regardless of the final configuration of the grid. Each position is connected to another by column. In the 3 highlighted rows, the digit is only a candidate to three positions. This is the basic configuration needed to be able to apply the Sudoku Swordfish strategy on rows. In every case, the digit cannot be a candidate to any other cell in the same row/column of the connected based cells. This is the theory, but there is good news! There is no need to test the possibilities because the result is always the same. The digit can then be safely deleted from all the cells that become impossible in all instances. The player can then test the different possibilities. Since the digit only has 2 or 3 possible positions in those row/columns and the cells are linked, it means the solution for that number will forcibly lie within that chain. Each cell must be connected to another by row/column, regardless of the shape created when they are linked. The grid must contain 3 rows or 3 columns where the digit is a candidate to only 2 or 3 cells. This means the player only needs to focus on one digit. The Sudoku Swordfish strategy is a single-candidate technique that uses 3 rows and 3 columns. Requirements to apply the Sudoku Swordfish strategy In these cases, the elimination of candidates is more restricted, but still possible. When one single constraint is missing, the Finned Swordfish or Sashimi Swordfish variants may apply. The grid needs to meet basic constraints in order to apply the Swordfish strategy. It can also allow the player to apply more basic and easier strategies to progress in the puzzle. The use of this strategy usually results in the unveiling of the solution of one or several cells. It is considered as basic due to the clarity of the fish pattern required for its application and its recurrence. If this post needs further clarification please let me know.The Sudoku Swordfish strategy is a basic advanced technique applied in the hardest levels of these puzzles to eliminate candidates. Quite simply, I have yet to find an easy way of spotting an x-wing without simply identifying four squares in a rectangular pattern and at most two 3x3 boxes that each have the same 2 possible numbers. Notice also that each of the locations highlighted in the original answer are in separate 3x3 boxes and may be solvable within those boxes. I recognize that this post is more complicated than the previous answer, but it is important to note that when identifying x-wings more than one number must be involved in order for it to actually be a standstill/unsolvable situation I have circled some of the other numbers involved in the x-wing predicament in the following image. So in either of the cases where the combos are 3&9 or 8&9 in the boxes, the x-wing forms (think about it - an x is made up of two lines) and the puzzles become unsolvable. The trick is that two different numbers are possible in each box involved in the x-wing (not just one, as the previous answer supposed - then, it may still become the only possible number, which is a crucial solving technique in sudoku). The issue is not only in the 'x-wing' shown before, but also in the three lowest boxes (7-9) in the shown columns 3 and 5. I am rehashing this question because I was going through some historical and it was poorly answered and understood before. Computers have no problem spotting them, of course. There can also be an X-Wing involving a row and a 3x3 box, but they are even more difficult to see. It is even more difficult to find 3 numbers in 3 rows/columns, or 4, etc. You'd have to look for 2 numbers occurring in 2 rows or columns. I usually don't look for them when solving with pen & paper. Imo, X-Wings are not easy to spot without highlighting all occurrences of a number. But then, for rows 3 and 8 to both contain a 9, Both B and D would have to be 9 and this is not possible, because then column 5 would have two 9's. If this were a 9, then A and C cannot be 9. Try making any of the red ones a 9, for instance row 1, column 3. So any 9's in these squares can be deleted. This means that any other squares in the same column as these squares can NOT be 9. In either case, these 4 squares will provide the 9's for rows 3 and 8, and because of that, they will also provide the 9's for their columns (3 and 5). Since every row must contain exactly one 9, either A and D must both be 9 OR B and C must both be 9. See how's there's two 9's in row 3 and also two 9's in row 8? (highlighted in blue) I've highlighted the X-Wing and the 9's involved in this X-Wing:
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