Naked Pair
A Naked Pair is two cells in the same unit that have exactly the same two candidates. The two digits must share these cells between them, so they can be removed from the rest of the unit.
Learn the techniqueHidden Pair
A Hidden Pair is two digits that can only go in the same two cells in a unit. Then the two cells must contain exactly these digits, and all other candidates in them can be removed.
See the technique in practice
Work through the examples step by step. Each step explains what you see on the puzzle and why the conclusion holds.
Example:
- We write candidate notes in all the empty cells on the puzzle and look more closely at row 7. At first glance it looks messy, but now we search for digits instead of cells.
- 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.
How to recognize the pattern
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.
Step-by-step procedure
- Choose a unit and go through the digits that are missing from it.
- Write for each digit which cells in the unit can take it.
- 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.
Common mistakes
- Searching for pure cells instead of confined digits. The hidden pair does not show up on the cell's candidate lists, only on the digit's possible placements.
- 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.
When do you need the technique?
At Medium level it is no longer enough to place digits directly. Now it is about removing candidates: when you can prove that a digit cannot go in a cell, the rest of the puzzle becomes easier. Write candidate notes, because that is the key to all techniques from here on.
Try it on your own puzzle
Enter your puzzle in the Sudoku Solver and it will find the next step and explain the technique behind it.
Open Sudoku Solver