Intermediate

Hidden Pair

Two digits that can go in only the same two cells of a unit — clearing all other candidates from those cells.

A hidden pair is the mirror image of a naked pair. Here, two digits can each be placed in only the same two cells of a unit. Those two cells must therefore hold those two digits, so every other candidate can be erased from them.

Hidden pairs are "hidden" because the two cells often carry many pencil marks; the pair is revealed only by tracking where each digit can live.

How to spot it

For a unit, note which cells can hold each digit. If two digits share exactly the same two candidate cells (and appear nowhere else in the unit), those cells form a hidden pair: keep only those two digits in them.

  • Track candidate cells for every digit in the unit.
  • Find two digits restricted to the same two cells.
  • Delete all other candidates from those two cells.
Only two cells of the box can take 4 or 9 — so they hold exactly {4,9}.

Worked example

  1. In a box, digit 4 can only go in cells A and B.
  2. Digit 9 can also only go in cells A and B.
  3. So {4,9} are locked into A and B.
  4. Cell A also showed {1,4,7,9}; cell B showed {4,6,9}.
  5. Erase everything but 4 and 9 from both cells, exposing further eliminations.

Try it yourself

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

Tap a cell, then a number, to practise.

Frequently asked questions

How is this different from a naked pair?
A naked pair is found from cells that already show only two candidates; a hidden pair is found from digits that can live in only two cells, even if those cells show more marks.
When should I look for hidden pairs?
After singles and naked pairs stall, and especially when several cells in a unit share a cluttered candidate set.

Related techniques

Further reading

Practice: Hidden Pair

Put the Hidden Pair to work on a live board — free puzzles with notes, hints and four difficulty levels.

Try it on a live board

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