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Hidden Singles

Beginner

A number that can only go in one cell within a row, column, or box.

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Example diagram — highlighted cells show where the technique applies.

How It Works

Hidden Singles occur when a particular number can only be placed in one cell within a row, column, or 3x3 box. Even though the cell may have multiple candidates, it is the only cell in its group that can hold that specific number.

Step by Step

  1. Choose a number (1-9) and a group (row, column, or box).
  2. Check all empty cells in that group for whether they can hold that number.
  3. If only one cell in the group can hold it, place it there.

When to Use

Use Hidden Singles when Naked Singles don't reveal any placements. Look for a number (1-9) that can only go in one cell within a row, column, or box, even if that cell has other candidates.

Example

In a row, the number 5 might be a candidate in three empty cells. But two of those cells are in a column that already has 5. So 5 can only go in the third cell — that's a hidden single for 5 in that row.

Common Mistakes

Only looking at cells with one candidate. Hidden singles are in cells that may have 2-3 candidates; the key is that one specific number has only one possible home in the group.

Tips

Systematically go through each number 1-9 and each row, column, and box. Ask: 'Where can this number go?' If the answer is exactly one cell, place it.

Practice This Technique

Try solving a puzzle and look for opportunities to apply Hidden Singles.

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