Naked single vs hidden single: two ways to find a Sudoku move

A naked single is a cell with only one possible digit. A hidden single is a digit with only one possible cell in a row, column or box. Both give a definite placement. The difference is which question you ask.

For a naked single, ask “what can go here?” For a hidden single, ask “where can this digit go in this unit?” A cell can have several candidates and still contain a hidden single.

Example 1: one digit fits this cell

Focus on r3c8. Row 3 has 3, 7 and 1, so its missing digits are 2, 4, 5, 6, 8 and 9. Column 8 already contains 2, 4, 1, 5 and 6, leaving 8 and 9 from that list. The top-right box contains a 9 at r2c7, so 9 is excluded too.

Only 8 survives. Put 8 in r3c8. You have combined three sets of restrictions on one cell. There is no need to decide where every other 8 in the puzzle belongs.

This is why a cell with one handwritten note is not automatically a naked single: the note must represent the complete list of candidates. If you forgot to write a possible digit, the apparent certainty comes from your notes, not the grid.

Example 2: only one cell accepts this digit

Row 3 in this different original grid reads blank, blank, blank, 4, 6, 7, 5, 1, 8. The missing digits are 2, 3 and 9. We want to place the row’s 2.

1. r3c1 cannot be 2 because column 1 already has 2 at r6c1.
2. r3c2 cannot be 2 because column 2 already has 2 at r8c2.
3. That leaves r3c3 as the only place for 2 in row 3.

Enter 2 at r3c3. Its own candidate list was 2,3,9, so looking only for cells with a single small digit would have missed this move. The row needs a 2, and there is nowhere else to put it.

The comparison in one glance

Find the next move

Return to the second grid. After r3c3 becomes 2, row 3 has only two blanks: r3c1 and r3c2. Which takes 3? Check column 1 before opening the answer.

Build a useful scanning habit

Alternate between the two questions when you pause. First check nearly filled cells for a single remaining candidate. Then pick a digit and inspect the rows, columns and boxes that still need it. You need not cover the whole board in notes before spotting a hidden single, but you do need to rule out every other location in the chosen unit.

The terminology follows HoDoKu’s singles guide; the grids and deductions here were created and checked for this article. See how to keep pencil marks current and the broader solving guide. For a fresh grid with manual notes and undo, explore Puzzle Lantern Sudoku.