Hidden pairs are naked pairs in reverse. Instead of looking for cells with two candidates, you look for two digits whose only landing spots are the same two cells.
How it works
Pick a row, column or box. For each digit that still needs a home in that unit, write down which empty cells could hold it. If two digits share exactly the same pair of candidate cells, those two cells must hold those two digits between them. Every other candidate inside those cells can be erased.
The cells may carry lots of other candidates, which is why these pairs are "hidden". You only see them by scanning digit positions, not by reading cell contents.
When to look for it
After naked pairs come up empty and the grid still has dense candidate sets. Boxes are the richest hunting ground because they have the most overlapping constraints. Start there.
Hidden pairs also show up in the late game when one digit dominates the remaining placements. Pick the digit you have already placed most often and check where its remaining copies can go.
Step-by-step example
- Pick a unit. A box with five or six empty cells is ideal.
- For every digit 1 through 9 still needed in the box, list the empty cells that could legally hold it.
- Find two digits with the same two-cell list. Say 4 can only go in r1c3 or r3c3, and 9 can only go in r1c3 or r3c3.
- Those two cells must hold {4, 9}. Erase every other candidate inside r1c3 and r3c3.
- The reduced cells often turn into a naked pair for the row or column they share, triggering more eliminations.
Hidden pair vs naked pair
Tips for spotting it
- Hidden pairs in boxes show up most often. Start there.
- Pick two digits that already appear in many of the nine units. Their remaining homes are tightly constrained.
- Once you find a hidden pair, the cells often collapse into a naked pair for another shared unit.
- Keep candidate marks up to date. A drifted pencil mark hides hidden pairs more than anything else.
Common mistakes
- Reading cell contents instead of digit positions.
- Stopping at the find. Strip the other candidates from the two cells. that is the whole point.
- Allowing a third cell to contain one of the pair digits. That breaks the pattern.
- Confusing hidden pairs with hidden singles. A single is one digit in one cell; a hidden pair is two digits in two cells.
Practise it
Play medium and hard sudoku with full pencil marks and force yourself to scan digit by digit through each box. After a few sessions the pattern starts jumping out before you have finished listing positions. Next up: hidden triples.