Scanning
The habit of sweeping the board for one digit at a time, using its existing placements to project elimination lines into empty regions.
A 5 in this column and a 5 in this row already block four cells of the box.
How to spot it
- 1Choose a digit already placed several times.
- 2For each box missing it, trace the rows and columns that already contain it.
- 3Those lines corner the digit — often into a single cell.
When to use
At the very start, and every time you place a new digit — rescan for the knock-on placements it enables.
Common mistake
Scanning haphazardly. Go digit by digit, most-frequent first, so nothing is missed.
Worked Example
Pick a digit that's already placed several times on the board — say 5, appearing four times already. For a box that's missing its 5, trace a line from each existing 5 through its row and column. Each line crosses into the box and blocks whichever cells it passes through.
Do this for all four placed 5s and, cell by cell, the box narrows down. If the lines between them cover every cell but one, that surviving cell is where the 5 belongs. This is the most basic and most-used technique in Sudoku — before reaching for anything fancier, always scan the digits you've placed the most of first, since they give you the most lines to work with.
From the Blog
- 7 Sudoku Mistakes That Keep You Stuck (and How to Fix Them)Guessing, skipping notes, scanning only boxes — the seven mistakes that stall improving players, with the exact fix for each one.
- Sudoku for Beginners: How to Solve Your First Puzzle, Step by StepNew to Sudoku? Learn the three rules, your first moves, and solve an easy puzzle step by step — with worked grid diagrams and no experience needed.
- The 5 Sudoku Techniques Every Beginner Should Learn FirstSkip the overwhelm. Master these five techniques — scanning, naked singles, hidden singles, cross-hatching, and pointing pairs — with worked grid diagrams for each.