Naked Singles and Hidden Singles, Step by Step

A naked single is a cell where only one digit can go. A hidden single is a digit that can only go in one cell of a row, column, or box. Between them, these two techniques solve nearly every Easy puzzle and most Medium ones.

They sound similar, but they come from opposite directions. A naked single starts from a cell and asks "which digits fit here?" A hidden single starts from a digit and asks "where can this go?" Good solvers switch between the two constantly. New to the rules? Start with How to Play Sudoku.

Naked singles: one cell, one option

Pick an empty cell. Look at its row, its column, and its box. Every digit that already appears in any of those three units is ruled out for this cell. If eight different digits are ruled out, the ninth is the answer.

Where to look: cells that sit where a crowded row meets a crowded column. The more givens a cell can "see," the more likely it's down to one option.

Why notes help: naked singles are easy to miss by eye because you have to hold 20 cells in your head at once. With candidates written in, a naked single is just a cell with one small digit left in it.

Hidden singles: one digit, one home

Pick a digit and a unit, say the digit 4 and the top-middle box. The box doesn't have a 4 yet, so it needs one somewhere. Check every empty cell in the box: can a 4 go there, or does its row or column already have a 4? If only one cell survives, the 4 goes there.

The cell may have other candidates too. That's what makes the single hidden: the cell doesn't look solved, but the digit has nowhere else to go.

Where to look: go digit by digit, 1 through 9, and check each box that's missing that digit. Then do the same for rows and columns, which are easy to forget. A row missing only two digits is a good place to find a hidden single.

Which to look for first

On Easy puzzles, scan for hidden singles first. You can find them without writing any notes, and they tend to open up the board fastest.

When scanning stops producing digits, write notes in the most crowded box or row and look for naked singles. Every digit you place creates new singles nearby, so after each placement, check its row, column, and box before moving on.

If both techniques run dry, the puzzle needs something stronger. The next step is pointing pairs, which uses a pattern inside a box to clear candidates out of a row or column.

Practice

• Naked Singles practice: eight cells, one technique.
• Hidden Singles practice
• Singles: Mixed Practice, where you have to recognize which one you're looking at.

Or play a Medium puzzle and count how many singles you can find before you need anything else.