Sudoku Clan › Sudoku techniques › Hidden pair
Hidden pair in Sudoku
- Advanced
- Needed from Expert puzzles
- 3 points in Duel
The pair that hides among other candidates.
When two digits can each go in only the same two cells of a unit, those two cells belong to them. Every other candidate in the two cells can be removed.
How it works
Say that in one row the digits 4 and 9 each have only two possible cells, and they are the same two cells. The row needs a 4 and a 9, and these are the only places for them, so the two cells will hold 4 and 9 in some order. Whatever other notes the cells have are impossible.
After the removal the two cells are a naked pair. The hidden pair is the same fact seen from the other side: a naked pair removes its digits from other cells, a hidden pair removes other digits from its own cells.
- Choose a unit with several empty cells.
- For each missing digit, count the cells it can go in.
- Find two digits that each have exactly two cells, and check that they are the same two.
- Erase every other note from those two cells.
A real example
The blue candidates are the two digits of the pair. In the highlighted unit they appear in those two cells only. The red candidates in the same cells are removed.
How to spot them
Hidden pairs live up to the name. The two cells usually carry four or five notes each and look like nothing special. The practical method is to scan by digit: in a unit, look for a digit that appears in exactly two cells, then see whether any other digit appears in exactly those two. Units with many empty cells are where hidden pairs occur. Units with only three or four empty cells usually show the same fact as a naked pair or triple instead.
What it gives you
The removal often leaves a different digit with only one place in a neighbouring unit, which is a hidden single. That is typical of advanced techniques: they do not place a digit themselves, they clear the way for a simple one.
Questions
What is a hidden pair in Sudoku?
Two digits that can only go in the same two cells of a row, column or box. Those cells must hold the two digits, so all their other candidates are removed.
Is a hidden pair harder than a naked pair?
It is harder to see, because the two cells have extra candidates around the pair. The logic is equally simple.