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Medium

Hidden Pair

A Hidden Pair is the Naked Pair's mirror image. Two digits, within one unit, can only go in the same two cells — even though those two cells might still show several other candidates on the surface.

Why it works

If 1 and 9 can only go in two specific cells within a row, those two cells must end up holding 1 and 9 between them, whatever else they currently show as possible. That means every other candidate in those two cells can be eliminated — they're not really live options once you account for where 1 and 9 have to land.

Worked example

5
3
7
6
1
9
5
9
8
6
8
6
3
4
8
3
1
7
2
6
6
2
8
4
1
9
5
8
7
9
Step 1 of 3

Look at the unit

In Row 7, 1 and 9 each have only two possible cells.

5
3
7
6
1
9
5
9
8
6
8
6
3
4
8
3
1
7
2
6
139
6
134579
2
8
4
1
9
5
8
7
9
Step 2 of 3

Find the pair

Both digits can only go in R7C1 and R7C3, so those two cells must hold 1 and 9.

5
3
7
6
1
9
5
9
8
6
8
6
3
4
8
3
1
7
2
6
139
6
134579
2
8
4
1
9
5
8
7
9
Step 3 of 3

Eliminate

Since 1 and 9 must go there, every other candidate in those two cells can be crossed out.

Tips

KaizenDoku spots every one of these patterns automatically after each game, and shows you exactly where you used them on your own board.

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More techniques

Naked SingleHidden SingleNaked PairPointing PairsBox Line ReductionX-WingSwordfishUnique Rectangle