Scanning in sudoku
Scanning is the first real technique in sudoku and the one you will use most at every level. The shift it asks for is small: stop staring at empty cells and start following one digit around the board. Placed digits block their rows and columns, the blocks pile up inside boxes, and boxes run out of room. Where a box has one cell left for a digit, you have a placement.
The sweep
Pick a digit and trace its placed copies. Below, the two 7s block the top two rows, and both rows pass through the top-right box. That box needs a 7 somewhere. Its top two rows are barred, and two cells of its bottom row already hold digits, so a single cell survives:
The placement follows with certainty. No pencil marks, no maybes, and the reasoning took three glances:
Making the habit fast
Start each scan with the digit that appears most on the board. Five or six placed copies leave few boxes unresolved, and every copy carves out a full row and column. On this site, tap any placed digit and all of its copies light up at once, so the sweep runs on highlights instead of memory. When a digit gives nothing, move to the next; easy boards refill with fresh scans after every placement.
Columns matter as much as rows. The eye reads sideways by habit, so a deliberate vertical pass catches the placements a row-only scan walks past.
When the scan comes up short
A scan can narrow a box to two cells and stop there. That is information, not failure:
Two survivors in one box is tomorrow’s move waiting. Another placement elsewhere often closes one of the cells, and on harder tiers that exact two-cell situation becomes raw material: locked candidates when the survivors share a line, strong links for the patterns above that.
Where scanning fits
Every easy puzzle on this site is verified to fall to scanning and singles alone, so this page plus the two single techniques is a complete easy toolkit. The sweep never retires, either. At every tier above, scanning is how you clear the cheap moves before the real work, and the digit-following instinct it builds is the foundation for hidden singles, where the counting gets one step finer.
Frequently asked questions
What is scanning in sudoku?
Scanning means following one digit instead of one cell. Its placed copies block their rows and columns, and where those lines cross a box, cells drop out. When a box has one surviving cell for the digit, you place it, no pencil marks required.
Which digit should I scan first?
The one with the most placed copies. A digit placed five or six times has few boxes left to resolve, and its lines carve up more of the board. On this site, tap any placed digit and its copies light up, which turns the choice into a glance.
Can you solve sudoku by scanning alone?
On our easy tier, yes. Every easy puzzle here falls to scanning and singles by design, verified when the puzzle is generated. From medium upward at least one moment requires candidate work, so scanning stays your fast tool without staying your only one.
Is scanning the same as cross-hatching?
Yes. Cross-hatching is the older name for sweeping a digit across rows and columns into a box, and some books use the two terms side by side. The habit matters, whatever it gets called.