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 6

5

9

1 6 7

8

4

1 3 7

2

1 3

7

8

2

5

1 9

3

1 4 9

6

4 9

4

1 6

3 6

1 7 9

2

6 7 9

1 3 7 9

5

8

5 8

3

2 5

1 5 8

9

7

4

6

1 2 5

6 8 9

2 5 9

5 6 7 9

1 4 8

1 2 5

4 5

1 9

3

5 7 9

5 7 9

4

1

6

3

2 5

5 7 9

8

2 5 7 9

2

7

8

5 9

1

6

3 5 9

4

3 5

6 9

4

5 6 9

3

5 9

8

2

7

1

1 3 5

1 9

3 5

2

7

4

8

6 9

5 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].