Two ways to find a certain answer

One option in a cell versus one home for a digit.

Look at one cell: naked single

A naked single has exactly one candidate left in a complete list. Every other digit is excluded by a rule or a previous deduction. Since the cell must contain something, place its sole candidate.

For exampleA cell whose complete list is {6} must be 6. No other cell needs to be nearly solved.

Look for one digit: hidden single

Pick a digit and scan one entire house. Ignore cells already filled and cells where that digit is ruled out. If only one location remains, that cell must hold the digit. The cell can still have several candidates: its other candidates are now unnecessary.

For exampleIn row 4, only r4c7 can hold 2. Even if r4c7 currently lists {2, 5, 8}, the row’s need for 2 forces it to be 2.

Why “hidden”?

The answer is hidden among other pencil marks in its cell. You reveal it by checking the digit’s locations throughout the house. This is a change in viewpoint, not a harder Sudoku rule. Try scanning boxes first, then rows, then columns.

For example“What can this cell be?” finds naked singles. “Where can this digit go in this house?” finds hidden singles.

Start a repeatable scanning loop

Look for a nearly complete house, inspect short candidate lists, then follow one digit through the boxes. After each placement, revisit its row, column, and box first. If nothing appears, it is fine to ask for a hint or learn the next idea; there is no need to guess.

For exampleFinish the reasoning aloud: “This row needs a 2, and every location except r4c7 is ruled out, so r4c7 is 2.”

Quick check

A cell contains candidates {2, 5, 8}. It is the only cell in its column that can hold 5. What follows?

Nothing, because the cell has three candidates.
Place 5: it is a hidden single in the column.
Place 2 because it is the smallest candidate.

Show the answer

Place 5: it is a hidden single in the column. Yes. This is one location for a digit, rather than one candidate in a cell.

Try it with the interactive lessons Play an easy puzzle

Next, when you are ready: Hidden single.