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This technique involves working out all the possible positions of a particular value by using the horizontal and vertical clues.
Now check to see if some of the possible candidates rule out others and thereby help you to fill in other cells. Even if this technique does not help you to complete more cells, it, nevertheless, allows you to narrow down the options. Look at the Sudoku puzzle opposite; all the possible positions for the number 5 are outlined in red.
In the central region, the 5 can only be placed in column 5, whereas in the lower central region, this same value can only go in row 8, either in column 4 or column 5. As the 5 of the central region can only be situated in column 5, we can rule out this column in the lower central region. The 5 in the lower central region must therefore be placed in row 8, column 4. We can now rule out all the other possible positions for the number 5 in the same row, column and region as the completed cell.
Put this technique to the test with one of our online Sudoku puzzles
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