Hidden Singles: Finding the Only Option

The Hidden Single technique is the perfect companion to naked singles. While naked singles look at what's left in a cell, hidden singles look at where a number can go within a region.

What is a Hidden Single?

A hidden single occurs when a number can only go in one place within a row, column, or box - even if that cell has multiple candidates. The placement is "hidden" among other possibilities.

How to Spot Hidden Singles

Look for a number that appears as a candidate in only ONE cell within a:

• Row
• Column
• Box (3x3 region)

Even if that cell has other candidates, you know this number must go there.

The Logic

If a number can only fit in one spot within a region, it must go there - regardless of what other numbers might also fit in that cell.

Example Scenario

Imagine row 5 needs the number 7. After checking all the constraints:

• Column 1: Already has a 7
• Column 2: Already has a 7
• Column 3: The cell could be 3, 5, or 7
• Columns 4-9: All blocked by other 7s

Even though the cell in column 3 has three candidates, you know 7 must go there because it's the only place in that row where 7 can fit.

Scanning Strategy

To efficiently find hidden singles:

1. Choose a number (e.g., start with 1)
2. Scan each row - Where can this number go?
3. Scan each column - Where can this number go?
4. Scan each box - Where can this number go?
5. Repeat for numbers 2-9

Common Mistakes

• Missing the forest for the trees: Focusing too much on individual cell candidates and missing region-wide patterns
• Not checking all three regions: A cell might have multiple candidates for its row, but be the only option in its box
• Confusing with naked singles: Remember - hidden singles focus on where a NUMBER goes, not what goes in a CELL

Why It's Called "Hidden"

The single is "hidden" because:

• The cell may have multiple candidates
• You need to look at the entire region to see it
• It's not immediately obvious like a naked single

Practice Tips

1. Pick one number and scan the entire grid for it
2. Look for boxes that are missing this number
3. Within those boxes, count how many cells could hold this number
4. If only one cell works, you've found a hidden single!

Combining with Naked Singles

These two techniques work beautifully together:

• Place a hidden single
• This often creates new naked singles
• Those naked singles reveal new hidden singles
• Keep alternating until you get stuck

Next Steps

Once you've mastered hidden singles, you're ready for intermediate techniques like Naked Pairs and Pointing Pairs that build on these foundational skills.