How to Find Hidden Singles

Scan by digit, not by cell

Choose a row and ask where a missing digit can go. Test each empty cell against its column and box. If every position except one is blocked, the remaining cell holds that digit. Repeat the same process for columns and boxes. This is different from looking for a cell with one candidate; you are looking for a candidate with one location.

For example, a row may have three unsolved cells with candidates {2, 5}, {2, 7}, and {2, 5, 7}. None is a naked single. If only the second cell can take 7 because another unit blocks 7 from the third, then 7 is a hidden single in the row.

Use boxes to narrow lines

Boxes are especially effective for hidden singles because their boundaries are compact. Pick a missing digit in a box and inspect the rows and columns crossing its empty cells. Existing copies of that digit in neighboring boxes often eliminate several positions at once. A location that looked ambiguous from the row alone may become forced when the box is included.

Record the reason clearly

When you place a hidden single, phrase the proof as “Digit 6 can appear only in R4C8 within row 4,” rather than “R4C8 looks like 6.” This habit separates valid deductions from intuition and makes it easier to diagnose mistakes. Sudoku0’s instructional hint panel follows the same observation-and-conclusion structure.

1. Choose one row, column, or box.
2. List the digits missing from that unit.
3. For each missing digit, mark every cell where a crossing unit blocks it.
4. If one legal position remains, place the digit there.
5. Update candidates and rescan the intersecting units.

Avoid false hidden singles

A candidate can appear only once in your notes because the notes are incomplete, not because the digit is truly forced. Hidden-single logic is valid only when all legal positions in the unit have been considered. If you use selective notes, recalculate the candidate in every empty cell of the unit before declaring a hidden single.