Hidden single in Sudoku

How it works

Pick a unit and a digit that is not in it yet. Each copy of that digit elsewhere on the board blocks the cells in its own row, column and box. If the blocking leaves just one open cell in your unit, the digit must go there. The cell may have room for other digits too, which is why the single is called hidden: you find it by asking where a digit can go, not what a cell can hold.

1. Choose a digit, ideally one that already appears many times.
2. Choose a box where it is missing.
3. Cross out, in your head, every cell of that box that shares a row or column with a copy of the digit.
4. If one empty cell is left, place the digit.

Example: a hidden single in a box

The circled digits are the copies that do the blocking. Follow each one along its row or column into the highlighted box and you will see that it rules out every empty cell but one. This sweep across the boxes is called cross-hatching, and it is how most people start every puzzle.

Example: a hidden single in a row or column

The same idea works along a line, and these are harder to see because a row is long and thin and your eye does not take it in at once. Here the blockers sit in the columns and boxes that cross the highlighted line. Puzzles rated Medium and above usually need at least a few of these.

Scanning tips

• Work one digit at a time across all nine boxes. It is faster than working one box at a time across all nine digits.
• Start with the digit that appears most often. More copies mean more blocking.
• When you place a digit, immediately check the same digit in the neighbouring boxes. One placement often unlocks the next.
• If boxes run dry, switch to rows and columns that are more than half full.

Hidden single or naked single?

They are two views of the same board. A hidden single asks where a digit can go inside a unit. A naked single asks which digits a cell can still take. Hidden singles are easier for people to see without notes, so look for them first.