Hidden Pair: learn the sudoku technique step by step at Sudokuen

See the technique in practice

Work through the examples step by step. Each step explains what you see in the puzzle and why the conclusion is valid.

Example:

We add candidate notes to every empty cell in the puzzle and take a closer look at row 7. It may look messy at first, but we are now searching by digit rather than by cell.
Look at the digits 1 and 9. In the entire row 7 only the two highlighted cells have them as candidates.
Then 1 and 9 must go in exactly these two cells, one in each. The cells are reserved for the two digits, even though the notes show more candidates.
Since the cells are reserved for 1 and 9, they cannot contain anything else. We remove all the other candidates from the two cells.

We add candidate notes to every empty cell in the puzzle and take a closer look at row 2. It may look messy at first, but we are now searching by digit rather than by cell.
Look at the digits 1 and 2. In the entire row 2 only the two highlighted cells have them as candidates.
Then 1 and 2 must go in exactly these two cells, one in each. The cells are reserved for the two digits, even though the notes show more candidates.
Since the cells are reserved for 1 and 2, they cannot contain anything else. We remove all the other candidates from the two cells.

We add candidate notes to every empty cell in the puzzle and take a closer look at column 9. It may look messy at first, but we are now searching by digit rather than by cell.
Look at the digits 2 and 6. In the entire column 9 only the two highlighted cells have them as candidates.
Then 2 and 6 must go in exactly these two cells, one in each. The cells are reserved for the two digits, even though the notes show more candidates.
Since the cells are reserved for 2 and 6, they cannot contain anything else. We remove all the other candidates from the two cells.

We add candidate notes to every empty cell in the puzzle and take a closer look at row 1. It may look messy at first, but we are now searching by digit rather than by cell.
Look at the digits 4 and 7. In the entire row 1 only the two highlighted cells have them as candidates.
Then 4 and 7 must go in exactly these two cells, one in each. The cells are reserved for the two digits, even though the notes show more candidates.
Since the cells are reserved for 4 and 7, they cannot contain anything else. We remove all the other candidates from the two cells.

We add candidate notes to every empty cell in the puzzle and take a closer look at row 9. It may look messy at first, but we are now searching by digit rather than by cell.
Look at the digits 4 and 6. In the entire row 9 only the two highlighted cells have them as candidates.
Then 4 and 6 must go in exactly these two cells, one in each. The cells are reserved for the two digits, even though the notes show more candidates.
Since the cells are reserved for 4 and 6, they cannot contain anything else. We remove all the other candidates from the two cells.

A Hidden Pair is two digits that can only go in the same two cells in a unit. Unlike the Naked Pair, the cells are often full of other candidates, which is precisely why the pair is called hidden. You find it by following the digits, not the cells: count how many cells each digit can go in.

When two digits are both confined to the same two cells, those two cells must contain exactly these two digits. All other candidates in the two cells can be removed, and often one of the cells becomes a Naked Single right after.

Choose a unit and go through the digits that are missing from it.
Note which cells in the unit can contain each digit.
Find two digits that both can only go in the same two cells.
Remove all other candidates from the two cells, so only the two digits remain.

Looking for clean-looking cells instead of confined digits. A Hidden Pair is not apparent from the cells' candidate lists; it appears only when you track the digits' possible positions.
Removing the pair's own digits. It is the other candidates in the two cells that should disappear, not the two digits that make up the pair.
Accepting a digit that also fits in a third cell. Both digits must be confined to exactly the same two cells.

At the Medium level, placing digits directly is no longer enough. The focus shifts to eliminating candidates: when you can prove that a digit cannot go in a cell, the rest of the puzzle becomes easier. Add candidate notes, as they are essential to every technique from this point on.

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Hidden Pair

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