Hidden Singles
What is a Hidden Single?
A Hidden Single happens when a number is forced into a cell because it's the only place in a row, column, or 3×3 box where that number can go. The "hidden" part comes from the fact that the cell may still have several candidates, but when you scan the whole unit, only one cell can legally contain that number.
This is different from a Naked Single, where the cell itself only has one possible candidate left. Hidden Singles often hide inside cells with three or four pencil marks — they're not obvious at first glance. Learn more about Naked Singles.
How to Spot Hidden Singles (Step by Step)
- Pick a row, column, or box.
- Focus on one number (let's say 6).
- Check each empty cell in that unit — can a 6 legally go there?
- If you find that only one cell can accept the 6, then that's your Hidden Single.
- Place the number, clear any conflicts, and repeat with other numbers.
Example Walkthrough
Example: In Row 4, scanning the unit shows that only the middle cell can accept the digit 6 based on the existing 6s in its columns and boxes. That single legal position is your hidden single—place 6 there.
Common Mistakes to Avoid
- Confusing it with Naked Singles: If a cell itself only has one candidate, that's naked. Hidden Singles are about a unit-wide restriction, not just the cell.
- Forgetting to scan both rows and columns: Many players only look at boxes. Hidden Singles often show up in rows and columns once the puzzle gets denser.
- Overlooking pencil marks: If you don't pencil in possibilities, you might miss that a number only has one legal spot.
Why Hidden Singles Matter
Hidden Singles are one of the most reliable ways to make progress when the grid seems stuck. They're often the bridge strategy: once you fill a few of them in, you'll uncover more Naked Singles and set up harder techniques like X-Wing or Swordfish.
Even advanced players rely on them constantly — they're the quiet workhorses of Sudoku solving.
Tips for Finding Hidden Singles Faster
- Scan systematically (row by row, column by column, then box by box).
- Look for units that are nearly complete — they're the most likely to reveal a hidden placement.
- Use pencil marks — they make Hidden Singles stand out much more clearly.
Practice Advice
When solving, don't just hunt for Naked Singles. After every placement, pause and check if you've unlocked a new Hidden Single. With practice, your eye will start spotting them automatically.
Wrap-Up
Hidden Singles may not look flashy, but they're essential. Think of them as the quiet clues that keep a puzzle moving forward. Mastering them means you'll never overlook an easy placement, and that strong foundation will prepare you for more advanced strategies.
Ready to advance? Check out our complete strategy guide for more techniques.
Frequently Asked Questions
What is a Hidden Single in Sudoku?
A Hidden Single happens when a number is forced into a cell because it's the only place in a row, column, or 3×3 box where that number can go. The "hidden" part comes from the fact that the cell may still have several candidates, but when you scan the whole unit, only one cell can legally contain that number.
How do I spot Hidden Singles?
To spot Hidden Singles: 1) Pick a row, column, or box, 2) Focus on one number (e.g., 6), 3) Check each empty cell in that unit to see if the number can legally go there, 4) If only one cell can accept the number, you've found a Hidden Single, 5) Place the number and continue with other numbers.
What's the difference between Hidden Singles and Naked Singles?
Hidden Singles are about a unit-wide restriction - a number can only appear in one specific cell within a row, column, or box, even if that cell has multiple candidates. Naked Singles occur when a cell itself only has one candidate left. Hidden Singles require scanning an entire unit to spot.
When should I look for Hidden Singles?
Look for Hidden Singles after every placement, especially after filling in Naked Singles. They often appear when the puzzle gets denser and are one of the most reliable ways to make progress when the grid seems stuck. They're the bridge strategy that leads to more advanced techniques.
How do Hidden Singles help solve puzzles?
Hidden Singles help solve puzzles by providing guaranteed placements that keep the puzzle moving forward. They often reveal new Naked Singles and set up harder techniques like X-Wing or Swordfish. Even advanced players rely on them constantly as they're reliable workhorses of Sudoku solving.
Practice Hidden Singles
Continue with Naked Pairs to boost your solving speed.