What is a hidden single in Sudoku?

A naked single is obvious: a cell has only one candidate, so you place it immediately. A hidden single is subtler: the cell has multiple candidates, but when you examine the unit it belongs to (row, column, or box), only one cell in that unit can hold a specific digit.

Example: digit 7 could go in cells A, B, or C within a box. But after applying row and column constraints, cells A and B cannot contain 7. Only cell C can. Even though C shows multiple candidates, 7 is its hidden single.

Work digit-by-digit through each unit. Take digit 3. Examine each box: in which cells can 3 go? If only one cell in the box allows 3 (all others blocked by a 3 in the same row or column), place it. Repeat for every digit in every box, then every row, then every column.

This scan is the core of what experienced solvers do almost automatically. The speed of this scan separates fast solvers from slow ones more than any other single skill.

Box scans produce the most hidden singles early in a puzzle, but rows and columns are equally valid. If digit 9 can only go in one cell within a row because all others are blocked by column or box constraints, place it immediately regardless of what other candidates that cell shows.

After exhausting naked singles (place every cell with only one candidate), scan for hidden singles before writing full pencil marks. Only add full candidates when both single types run dry. This order minimises the time you spend maintaining and updating pencil mark sets as digits are placed.