Scanning is the core skill in Sudoku solving. It's how you find hidden singles quickly — by systematically checking where each digit can go in rows, columns, and boxes.
Quick Summary
What: Systematically checking where digits can be placed
How: Cross-hatch rows and columns to eliminate positions
Goal: Find cells where only one digit can go, or digits that can only go in one cell
Difficulty: Beginner — the first real skill to develop
The Concept
Scanning is about asking two questions repeatedly:
Where can this digit go? (in a row, column, or box)
What can go in this cell?
By visually tracing lines from existing digits, you eliminate cells until only one possibility remains. This is faster than writing out all candidates — it's mental elimination.
Cross-Hatching
✖️ Cross-Hatching
For a specific digit, visualize lines extending from every instance of that digit on the grid. Cells crossed by these lines cannot contain that digit. In a box, the remaining uncrossed cells are the only possibilities.
Cross-hatching for digit 5:
The ✓ at R6C3 is the only uncrossed cell for 5 in Box 4.
How to Scan
1
Pick a digit (1-9)
Start with digits that appear frequently — they create more elimination lines.
2
Pick a box to analyze
Choose a box where that digit hasn't been placed yet.
3
Trace elimination lines
From every instance of your digit in the grid, trace horizontal and vertical lines. These cells are eliminated.
4
Count remaining cells
In your target box, how many cells are NOT crossed out? If only one, place the digit!
5
Repeat
Move to the next box. Then try a different digit. Keep cycling.
Row and Column Scanning
Don't just scan boxes — scan rows and columns too:
Row Scanning
For a specific digit, check if it can only go in one cell within a row.
Cross-hatch using column and box constraints.
Column Scanning
For a specific digit, check if it can only go in one cell within a column.
Cross-hatch using row and box constraints.
Scanning Efficiently
Start with High-Frequency Digits
If 7 already appears 6 times on the grid, there are more elimination lines. The remaining 7s are easier to find.
Focus on Crowded Areas
Boxes or lines with many filled cells have fewer possibilities. Scan these first — you'll find placements faster.
Integrate Snyder Marking
When you scan and find a digit that can only go in 2 cells within a box, mark it with Snyder Notation. This saves information for later.
Pro Tips
Scan in Passes
Do a complete pass for digit 1, then digit 2, etc. Or scan all boxes for your chosen digit before switching. Systematic scanning prevents missing easy placements.
Rescan After Placements
Every placement changes the grid. Immediately rescan the affected row, column, and box for new opportunities.
Mental vs Written
Experienced solvers scan mentally without writing candidates. Beginners should write candidates (pencil marks) to avoid mistakes. As you improve, you'll need fewer written marks.
This IS Hidden Singles
Scanning is how you find hidden singles. When scanning reveals only one cell for a digit, that's a hidden single!
Intermediate