Hidden Single

Level: first needed in Easy puzzles. In one line: A number that fits in only one cell of a row, column or box.

A Hidden Single is a number that has only one possible place inside a row, column or box. The cell itself might still have several candidates, which is why the single is "hidden", but the number has nowhere else to go, so it must go there.

Hidden singles are the workhorse of Sudoku. Easy and Medium puzzles in Sudoku Arena can be solved with nothing else, and they are usually the first thing to look for in harder puzzles too.

Example from a real puzzle

This position comes from an Easy puzzle in Sudoku Arena, at the exact moment the Hidden Single is the next logical move. Cells are named by row and column: R3C4 means row 3, column 4.

1 can only go in R3C2 within box 1. The highlighted cell R3C2 can only be 1.

Play this puzzle and look for the move yourself. It's a practice game, so it doesn't count towards your stats.

How to spot it

1. Choose a number, ideally one that is already placed many times.
2. Go box by box. For each box that does not contain the number yet, look along the rows and columns that pass through it.
3. Every row or column that already holds the number rules out the cells it crosses in that box (this is called cross-hatching).
4. If only one empty cell in the box is left, the number goes there.

Tips

• Work through the numbers 1 to 9 in order, then start again: placing one number often reveals another hidden single straight away.
• Check rows and columns too, not only boxes. A row with a single gap for a number is just as good.

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