Hidden Singles
Find numbers that can only appear in one cell within a row, column, or 3×3 box.
What Are Hidden Singles?
A Hidden Single occurs when a specific number can only be placed in one cell within a row, column, or 3×3 box, even though that cell might have multiple possible candidates. The number is "hidden" among other possibilities.
Key Difference from Obvious Singles
- • Obvious Single: Only one number can fit in a cell
- • Hidden Single: Only one cell can contain a specific number
Why "Hidden"?
The number appears "hidden" because the cell may seem to have multiple options, but when you focus on where that specific number can go in the region, only one location is possible.
How to Find Hidden Singles
The Method
- 1 Pick a specific number (1-9)
- 2 Choose a row, column, or 3×3 box to examine
- 3 Check if that number already exists in the region
- 4 If not, find all empty cells where it could go
- 5 If only one cell is possible, you found a Hidden Single!
Systematic Approach
Work through each number systematically:
Check each number in each row, column, and box
Step-by-Step Example
Step 1: Choose a Number and Region
Let's look for the number 4 in the top-left 3×3 box (highlighted in blue).
Question: Where can the number 4 go in this box?
Current state: The top-left box contains: 5, 3, 6, 9, 8
Missing numbers: 1, 2, 4, 7
Top-left box highlighted in blue
Step 2: Check Constraints for Number 4
Now let's see which empty cells in the top-left box can contain the number 4:
Row 1, Column 3
Check constraints:
- • Row 1: Has 5, 3, 7 (4 is possible)
- • Column 3: Has 8, 6, 8 (4 is possible)
- • Box: Has 5, 3, 6, 9, 8 (4 is possible)
✓ Cell can contain 4
Row 2, Column 2
Check constraints:
- • Row 2: Has 6, 1, 9, 5 (4 is possible)
- • Column 2: Has 3, 9 (4 is possible)
- • Box: Has 5, 3, 6, 9, 8 (4 is possible)
✓ Cell can contain 4
Row 2, Column 3
Check constraints:
- • Row 2: Has 6, 1, 9, 5 (4 is possible)
- • Column 3: Has 8, 6, 8 (4 is possible)
- • Box: Has 5, 3, 6, 9, 8 (4 is possible)
✗ Wait, column 3 has 4 in row 7!
Step 3: Eliminate Based on Column Constraint
Looking at column 3, we can see that the number 4 already appears in row 7. This means no other cell in column 3 can contain 4.
Eliminated: Row 1, Column 3 and Row 2, Column 3
Both cells are in column 3, which already has a 4
Hidden Single Found!
The number 4 can only go in Row 2, Column 2 within the top-left box
Green: Hidden Single found | Red X: Eliminated positions | Yellow: Other 4s
Types of Hidden Singles
Row Hidden Single
A number can only appear in one cell within a specific row
Column Hidden Single
A number can only appear in one cell within a specific column
Box Hidden Single
A number can only appear in one cell within a specific 3×3 box
Tips for Finding Hidden Singles
Effective Strategies
- ✓ Start with numbers that appear frequently in the puzzle
- ✓ Focus on regions (rows/columns/boxes) with few empty cells
- ✓ Work systematically through numbers 1-9
- ✓ After placing a number, check for new hidden singles
Common Mistakes
- ✗ Only checking one constraint (row, column, or box)
- ✗ Forgetting to check all empty cells in a region
- ✗ Confusing hidden singles with obvious singles
- ✗ Not rechecking after placing new numbers
Practice Exercise
Can you find the hidden single?
Look at the middle row (row 5) in this puzzle. Can you find a hidden single for any number?
Hint: Look for the number 2 in row 5.
Check which cells in row 5 can contain the number 2, considering column and box constraints.
Row 5 highlighted - find where 2 can go!
Master Hidden Singles with Practice
Hidden Singles are one of the most important techniques in Sudoku. Practice finding them in our game!
Practice Hidden Singles