In a Sudoku puzzle, if two cells in a 9-cell unit (a row, a column, or a 3x3 block) have only two digits as candidates and the two digits are exactly the same, then these two digits must each occupy one of these two cells. They cannot occupy other cells in the same 9-cell unit. So, if these two digits appear in other cells in the same 9-cell unit as candidates, they can be removed.
The Naked Pair technique can be used in a row, in a column, or in a 3x3 block. The following examples will use real Sudoku puzzles to demonstrate this technique in all these three situations.
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In row R2 of the puzzle in Figure 1a, the two green cells (R2,C2) and (R2,C5) have the same two candidate digits 5 and 7, which are the only candidates for the two green cells. Therefore, the digits 5 and 7 each will occupy one of these two green cells and cannot occupy any other cells in the same row. Consequently, the candidates 5 and 7 in other cells in the same row can be removed, as shown in Figure 1b below.
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In column C4 of the puzzle in Figure 2a, the two green cells (R3,C4) and (R5,C4) have the same two candidate digits 4 and 7, which are the only candidates for the two green cells. Therefore, the digits 4 and 7 each will occupy one of these two green cells and cannot occupy any other cells in the same column. Consequently, the candidates 4 and 7 in other cells in the same column can be removed, as shown in Figure 2b below.
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In block B3 of the puzzle in Figure 3a, the two green cells (R1,C8) and (R3,C7) have the same two candidate digits 5 and 8, which are the only candidates for the two green cells. Therefore, the digits 5 and 8 each will occupy one of these two green cells and cannot occupy any other cells in the same block. Consequently, the candidates 5 and 8 in other cells in the same block can be removed, as shown in Figure 3b below.
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List of Sudoku Solving Techniques
Previous: Locked Candidates
Next: Naked Triplet