Scanning is a search skill: you inspect the grid with one clear target instead of staring at all 81 cells at once.
You might scan:
- a nearly complete row, column, or box;
- one digit across several units;
- one promising empty cell;
- the units changed by your last placement.
Scanning itself is not the deduction. It is the method you use to find deductions such as a last missing digit, Hidden Single, or other constrained placement.
Why scanning works
Every unsolved cell is constrained by three units:
- its row;
- its column;
- its 3×3 box.
Every placed digit also restricts where that same digit can appear in related units.
Scanning works by repeatedly using those constraints to reduce possibilities until something becomes forced.
The logic is simple. The skill is learning where to look next.
Scan method 1: nearly complete units
Start with rows, columns, or boxes that have only a few empty cells.
Suppose a row contains:
1 2 3 4 5 6 7 . 9
The only missing digit is 8, so the empty cell must be 8.
That is a last missing digit / last missing digit.
With two or three empty cells, list the missing digits and use the crossing columns and boxes to see which cell can accept each one.
Scan method 2: one digit at a time
Choose a digit, such as 5, and find every placed 5 in the grid.
Now inspect a box that does not yet contain 5. Rows and columns containing existing 5s block cells in that box.
If only one legal cell remains, 5 must go there.
This targeted digit scan is one of the cleanest ways to find Hidden Singles and leads directly into crosshatching.
Scan method 3: one box, one digit
Choose one digit and inspect a box that still needs it.
Use existing copies of that digit in crossing rows and columns to rule out positions inside the box.
If only one cell survives, you have found a Hidden Single.
This focused method is crosshatching.
Scan method 4: promising cells
If you are looking for a Naked Single without full candidate notes, choose a cell whose row, column, and box already contain many different digits.
Collect the digits visible in its peers. If eight of the nine digits are blocked, the remaining digit is forced.
This cell-focused scan is different from a digit-focused Hidden Single search, and learning to switch between those viewpoints is useful.
A simple scanning order
For a beginner puzzle:
- check nearly complete boxes;
- check nearly complete rows;
- check nearly complete columns;
- choose one digit and scan all boxes for that digit;
- repeat with other digits;
- after every placement, re-check the affected row, column, and box.
You do not have to follow this exact order forever. The purpose is to remove randomness while the habit develops.
What is crosshatching?
Crosshatching is a specific form of digit scanning.
You focus on one digit and use existing copies in rows and columns to block positions inside a box. If the blocks leave only one cell, the digit is placed there as a Hidden Single.
The dedicated Crosshatching Guide breaks that process down visually.
Scanning vs candidates
Scanning can be done without writing every candidate.
That makes it ideal early in a puzzle, when many placements are still visible from the large structure of the grid.
When scanning stops producing progress, candidates make the remaining information explicit and let you use more systematic eliminations.
A useful progression is:
scan → Singles → candidates → eliminations → scan again
not:
fill every candidate immediately → never look at the big grid again.
Common scanning mistakes
Scanning without a target
Move with a purpose: one unit, one digit, or one cell. Unstructured staring creates fatigue.
Ignoring one unit type
A cell can look valid from its row but be blocked by its box. Always remember row, column, and box constraints interact.
Not re-scanning after a placement
Every solved digit changes three units and may unlock several simple moves.
Looking only for placements
At intermediate levels, scanning may reveal candidate confinement rather than a solved digit. That still matters.
Moving too quickly through digits
If you choose digit 7, actually inspect its relevant boxes and lines before jumping to 8. A systematic pass is more reliable than a visual skim.
FAQ
Is scanning a formal Sudoku technique?
The word is often used broadly for systematic visual searching rather than one single deduction rule. VeyraPlay uses it as the foundation skill that includes unit scans, digit scans, and other simple visual checks.
Can scanning solve a whole Sudoku?
Many Easy puzzles can be solved largely through repeated scanning and Singles. Harder puzzles eventually require candidate eliminations and more advanced techniques.
What should I scan first?
Nearly complete units are a good starting point. After that, scanning one digit across boxes can reveal Hidden Singles efficiently.
Do I need pencil marks for scanning?
No. Basic scanning can be done directly from solved digits. Candidates become more useful when the puzzle stops yielding obvious placements.
What to learn next
Learn Crosshatching for a focused digit-scanning method, then study Naked Singles and Hidden Singles so you can name the deductions your scans reveal.