Hidden pairs in sudoku
A naked pair announces itself: two cells, two candidates, done. The hidden pair buries the same structure under extra candidates, and digging it out is a defining skill of hard puzzles. Two digits that fit nowhere in a unit except the same two cells own those cells, whatever else is penciled beside them.
The pattern
Work inside one unit and count homes per digit. In the center box below, the 1 can go in two cells, and the 9 can go in the same two cells. Between them, those two digits must fill both. The 5 sharing a cell with them, and the 4 sharing the other, are candidates for cells that are already spoken for.
Check the count yourself. The box holds five placed digits and four open cells. The two open cells on the right show 4 and 5 only, and the board explains why: the placed 1 above them and the placed 9 below them stand in their shared column, so neither cell can take a 1 or a 9. That leaves the two marked cells as the only homes for both digits.
The elimination
Two digits, two cells, a private arrangement. The pair cells will take the 1 and the 9 in some order, which means the 5 and the 4 penciled inside them can never come true. Erase them:
Notice the direction. A naked pair cleans the rest of its unit; a hidden pair cleans the inside of its own two cells. The payoff feels smaller on the board and lands just as hard, because stripping a pair cell to two candidates turns it into raw material for pairs, wings and chains further down the solve.
How to hunt
Counting is the entire method. Pick a unit that has stalled, and for each unplaced digit, count its possible cells. A digit with exactly two homes goes on your shortlist. Two shortlisted digits sharing both homes are a hidden pair, and anything else written in those two cells is removable.
Boxes make the comfortable hunting ground because their cells sit together visually. Rows and columns hide pairs just as often, and the same count uncovers them. On this site, select any placed copy of a digit to light up its remaining candidates, then count the lit cells inside the unit you are working.
Near misses
A hidden pair dies the moment either digit picks up a third home. Same box as before, one placed digit removed:
The 9 still has two homes here, and that alone earns nothing. Both digits of a pair must be confined to the same two cells, so one loose digit sinks the whole claim. Stale pencil marks cause the opposite failure: a candidate you forgot to erase makes a digit look looser than it is, and you walk past a real pair. Run Auto notes before a serious hunt and the picture stays truthful.
Triples, and where this leads
Three digits confined to three cells of a unit form a hidden triple, and the counting method scales without changes. Hidden sets are the workhorse eliminations of our hard tier, sharing that rung with the X-wing, and the counting habit they build is the same one that later finds strong links for skyscrapers and coloring.
Frequently asked questions
What is a hidden pair in sudoku?
Two digits that fit in only the same two cells of a row, column or box. Those two cells must take those two digits between them, so every other candidate penciled in either cell is noise you can erase. The pair hides because the extra candidates bury it.
What is the difference between a naked pair and a hidden pair?
A naked pair shows itself: two cells holding exactly two candidates each. A hidden pair sits under clutter, and its cells can show three or four candidates. The eliminations differ too. A naked pair cleans the rest of the unit; a hidden pair cleans the inside of its own two cells.
How do you find hidden pairs?
Count homes, one digit at a time, inside one unit. Any digit with exactly two possible cells is half of a potential pair, so check whether another digit shares both of those cells. The counting feels slow the first few times and becomes a glance with practice.
Do hidden pairs appear in rows and columns too?
Yes. Boxes make the easiest hunting because their nine cells sit together visually, but the logic holds in any unit. Hidden triples follow the same rule with three digits confined to three cells, extra candidates and all.