A hidden pair sits inside a row, column or box where two digits can only go in the same two cells, even though those two cells still show other candidates in their pencil marks. The pair is "hidden" because it is buried under extra notes that have not been ruled out yet. Once you spot it, you can strip those extra candidates away and the two cells collapse down to exactly the digits in the pair.
This is the natural next step after naked pairs, which look almost identical but from the opposite direction. A naked pair is two cells that already show only two candidates each. A hidden pair is two cells that could show four, five or more candidates, with the real answer buried inside because two digits happen to be trapped in exactly those two spots.
Hard puzzles on this site (28 to 32 givens) lean on this move once naked and hidden singles run out; a Medium board rarely needs it. Turn on Notes and watch for two digits that keep appearing together in the same two cells of a row, column or box.
Worked example
This is a real puzzle position with one solution. Step through to see a hidden pair uncovered in row 5.
How to find a hidden pair
Work one unit at a time: a row, a column or a box. For each unit, list which cells still hold a given digit as a candidate. If two digits both live in exactly the same two cells and nowhere else in that unit, you have found a hidden pair.
- Check digits, not cells. Naked pairs jump out from a cell's own pencil marks. Hidden pairs only show up when you scan a unit digit by digit: where can 4 go? Where can 7 go? If the answer is the same two cells for both, that is the tell.
- Ignore how crowded the cells look. A cell with five or six pencil marks can still be half of a hidden pair. The extra marks do not disqualify it; they are exactly what you are about to erase.
Once found, remove every other candidate from those two cells. The pair itself does not get placed yet, but the row, column or box is now one step closer to a naked single somewhere else, since other cells lost a possible home for those two digits.
Hidden pair or naked pair?
Both patterns remove the same kind of clutter, but they start from different places. A naked pair is two cells whose pencil marks already show only two digits, so anything else in the unit that could also take those digits gets crossed out. A hidden pair starts from two digits that are trapped in two cells, even if those cells show four or five marks each.
In practice, a naked pair cleans up other cells in the unit. A hidden pair cleans up the two cells you found. Some solvers scan for naked pairs first because they are easier to see, fewer marks stand out, then look for hidden pairs when the board stops giving up naked ones.
Common mistakes
- Stopping at "these two digits share a cell." That is not enough. Both digits need to be confined to the same two cells and nowhere else in the unit. If digit 4 also has a candidate in a third cell, there is no hidden pair yet.
- Forgetting to erase the other candidates. Finding the pair is only half the job. If you spot that 2 and 7 are trapped in two cells but leave the 3 and 6 marks sitting there, you gain nothing. On this site the Notes button shows every mark, so cross out anything that is not part of the pair.
- Mixing up rows, columns and boxes. A hidden pair must sit inside one unit. Two cells that share a digit but live in different rows, columns and boxes are not a pair, they are just two cells.
When you will actually need this
Easy and most Medium puzzles never need this move; naked and hidden singles carry them the whole way, sometimes with help from pointing pairs. Hidden pairs start earning their keep on Hard puzzles, which ship with 28 to 32 givens and are built so the easier techniques stall partway through.
When you hit a wall on a Hard board, do not guess. Switch on Notes, pick a box that looks stuck, and check each digit 1 to 9 against the empty cells in that box. If two of them keep landing in the same two cells, you have your hidden pair and a way forward without touching the Hint button.
Hard boards carry 28 to 32 givens, few enough that singles alone stop working. Turn on Notes and hunt for two digits sharing two cells.
Frequently asked questions
A hidden pair is two cells in the same row, column or box where two digits can only go, even though the cells still display other candidates. Removing the other candidates leaves just the pair.
A naked pair starts from two cells that already show only two candidates each. A hidden pair starts from two digits trapped in two cells that may still show several other candidates until you clean them up.
Yes, in practice. Hidden pairs are hard to spot in your head because the cells look crowded. Turning on Notes and checking each digit 1 to 9 against a unit is the reliable way to find them.
No. It only removes extra candidates from the two cells in the pair. That cleanup often creates a naked single somewhere else in the row, column or box on a later move.
Hard puzzles, which have 28 to 32 givens, are the first level where naked and hidden singles regularly run out and hidden pairs, along with pointing pairs and naked triples, are needed to keep progressing.