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Basic Techniques

Line and Box Intersections

Subset Techniques

Seafood Techniques

Chain Techniques

Other Techniques

Hidden Pairs

If you find two cells in a row, column or block that are the only two cells to contain two candidate numbers (they may contain other numbers as well, but these are the only two cells that contain these two numbers) then you can remove all the other numbers from those two cells.

Because these two cells are the only places that these two numbers can appear, and they have to appear in this row, column or block, the one cell must be one of the numbers, and the other cell must therefore be the other. Neither cell can take any other number, because these two number have to appear somewhere in that house.

1 3 659
1 6 784
1 3 721 3
782
51 93
1 4 964 9
41 63 6
1 7 926 7 9
1 3 7 958
5 832 5
1 5 897
461 2 5
6 8 92 5 95 6 7 9
1 4 81 2 54 5
1 935 7 9
5 7 941
632 5
5 7 982 5 7 9
278
5 916
3 5 943 5
6 945 6 9
35 98
271
1 3 51 93 5
274
86 95 6
Example 1: Hidden Pairs
Hidden pair cells

If we look at column 9, the numbers 7 and 9 only appear in two cells, [r2,c9] and [r6,c9]. Because there are two numbers, and they only appear in two cells, these two cells must contain these numbers, so we can remove 6 from [r2,c9], and 2 and 5 from [r6,c9].

Keywords

block, candidate, cell, cells, column, hidden pairs, house, row