Hidden Pair
A Hidden Pair is the flip side of the naked pair. A naked pair is two cells that hold only two candidates; a hidden pair is two digits that can go in only two cells — but those cells are cluttered with other candidates, so the pair is hidden in plain sight. Learn to spot it and you can clear away that clutter in one move.
What is a Hidden Pair?
Look at a unit (row, column or box) from the digits' point of view. If two numbers — say 4 and 7 — can each go in only the same two cells of that unit, and nowhere else, then those two cells must hold the 4 and the 7 between them. Every other candidate in those two cells can be deleted.
It's called hidden because the pair doesn't stand out: each cell still shows extra candidates, and you only find the pair by tracking where each digit is allowed to go.
Step by step
- Fill in all pencil marks. Hidden pairs are impossible to see without a complete candidate list.
- Pick a unit and go digit by digit. For each number, note which cells it can still go in.
- Find two digits that share exactly two cells. If 4 goes only in cells A and B, and 7 goes only in A and B too, that's a hidden pair.
- Strip the extras. Remove every candidate except those two digits from cells A and B — they collapse to a naked pair.
- Re-scan. The cleared candidates often reveal singles elsewhere.
The digits 4 and 7 appear in no other cell of the row — only the two indigo cells. So those two cells are {4, 7}: delete the 2 from the first and the 9 from the second.
Hidden pair vs naked pair
They describe the same situation from opposite directions. A naked pair jumps out because the two cells show only two candidates. A hidden pair is harder — the cells look busy, and you have to notice that two digits have run out of anywhere else to go. If you've mastered naked pairs and hidden singles, this is the technique that combines both ideas.
Common mistakes
- Stopping at the first digit. One digit confined to two cells is nothing on its own — you need a second digit confined to the same two cells.
- Deleting the wrong candidates. The pair's own two digits stay; everything else in those two cells goes.
- Only checking cells, not digits. Naked subsets are found by scanning cells; hidden ones are found by scanning digits. Get in the habit of both.
Which Sudoku types is this best for?
The same logic applies across variants, but it pays off more in some than others.