Lesson:Scanning Technique
Scanning is your primary tool. Let's look for the number 1.
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The Scanning Technique — also known as cross-hatching — is the most fundamental solving strategy in Sudoku and the first method every beginner should master. It involves systematically looking for a specific digit across rows and columns to determine the only possible position for that digit within a given 3×3 box. Scanning is the foundation upon which all other solving techniques are built.
The technique is called cross-hatching because you mentally draw horizontal and vertical lines through the rows and columns that already contain the target digit. The cells in your target box that fall on these imaginary lines are immediately eliminated — the target digit cannot go there. If only one cell in the box remains uncrossed, you have found a definite placement.
Experienced solvers scan automatically and rapidly, sweeping their eyes across the board with trained efficiency. Developing a systematic scanning rhythm — working through each digit from 1 to 9, checking each box — is one of the most valuable skills you can build as a Sudoku solver.
How It Works — Step by Step
Step 1 — Choose a digit to scan for
Pick a digit, say 5. You want to find where the 5 goes in boxes that do not yet have a 5. Start with boxes that already have many filled cells — they will have fewer candidates to consider.
Step 2 — Find rows and columns that already contain the digit
Look across the entire board. Mark (mentally or with your eyes) every row and column that already contains your chosen digit. These rows and columns "block" the digit from appearing in their intersections within your target box.
Step 3 — Eliminate blocked cells in the target box
In your target box, cross out every cell that lies in a blocked row or blocked column. Any cell in the same row as a 5 elsewhere on the board cannot hold a 5. Same for columns.
Step 4 — Place the digit if only one cell remains
If only one cell in the box remains unblocked, that is where the digit must go. Place it immediately. If two or more cells remain, the scanning technique alone cannot resolve this box for this digit — move to the next digit or the next box.
When to Use This Technique
Use scanning at the very start of every puzzle, before any other technique. It is fast, requires no pencil marks, and often fills 20–40 cells on Easy puzzles. Return to scanning after every placement — new placements change which rows and columns are blocked and may create new scanning opportunities.
Worked Examples
You are scanning for digit 7. You see 7s already placed in: Row 1, Row 4, Row 7, Column 2, Column 5, Column 8. Looking at the middle-left box (rows 4–6, columns 1–3): Row 4 is blocked (has a 7). Column 2 is blocked. Column 3 is free. The remaining cells at R5C1, R5C3, R6C1, R6C3 — but R4 is blocked, so R5C1, R5C3, R6C1, R6C3 remain. Column 2 blocks nothing new here... check again. If scanning fully blocks all but R6C3, place 7 there.
Frequently Asked Questions
What is the difference between scanning and looking for Naked Singles?
Scanning focuses on placing a specific digit in a specific box by elimination from rows and columns. Naked Singles focus on a specific cell that has only one remaining candidate. Both are essential beginner techniques; scanning is usually faster as a first pass.
Should I always scan all nine digits before moving to another technique?
Not necessarily. Scan each digit across all boxes, then repeat the cycle. After new placements, earlier scans may become productive again. A good rhythm is: scan all digits, place all possible, repeat until no more scanning placements are found.
Can scanning work on 6×6 or 4×4 puzzles?
Absolutely — the cross-hatching logic applies to any grid size. For a 4×4 Sudoku, you scan digits 1–4 across 2×2 boxes. For 6×6, you scan digits 1–6 across 2×3 boxes.
Ready to test your knowledge? Try applying this technique in our Easy Sudoku puzzles, Beginner level course or explore Medium Sudoku puzzles. Keep training to improve your solve times and master the grid!
Ready to Practice?
Apply this technique on a real puzzle from our daily or practice modes.