namlabsWhat is a hidden single?
Every row, column and box needs each digit from 1 to 9 once. If a digit can go in only one cell of a house, it must go there, even when that cell still has several candidates of its own. The single is hidden among the other candidates, which is why you find it by looking at a house, not at a cell.
Example
Where can 5 go in the top-left box? Row 2 already has a 5 (in R2C6), so no cell of row 2 in this box can take it. Row 3 has a 5 in R3C8, which rules out row 3. Column 2 has a 5 in R7C2, which rules out R1C2, and R1C1 is filled. Only R1C3 is left, so the 5 goes there.
Looked at on its own, R1C3 could still be any digit except 8. You would not find this 5 by looking at the cell alone.
How to find hidden singles
- Pick a digit, ideally one that already appears many times on the board.
- Pick a box that does not have it yet.
- Rule out every cell that is filled or that shares a row or column with that digit.
- If only one cell is left, place the digit. Then try the next box.
This is the scanning most players do without naming it. The same check works for rows and columns.
Where it fits
Hidden singles are the most common move in easy and medium puzzles. Pair them with naked singles; when both run out, move on to naked pairs.
Practise on your phone
Our free Sudoku app has hints that point out the technique you can use next, with the cells highlighted. It plays offline.
