Scanning is the first method almost every Sudoku player learns, usually without being taught it directly. You pick a digit, say 8, and look at where it already sits on the board. Then you trace the rows and columns running through those 8s into a box that does not have one yet. Wherever those lines cross, 8 is blocked. What is left over is often just one cell.
Puzzle books call this cross-hatching because the lines you are tracking through a box, drawn from the rows and columns that already hold the digit, form a crosshatch pattern over the empty cells. No pencil marks are needed. You are not listing every candidate for every cell, you are chasing one digit at a time and letting the grid rule out cells for you.
This is how most placements happen on Easy Sudoku boards, where 40 or more of the 81 cells are already filled in. With that many givens, a digit typically has strong rows and columns crossing most boxes already, so scanning alone clears a large share of the puzzle before you need to think about candidates at all.
Worked example
A real puzzle, mid-solve. Step through finding where 8 goes in the top-middle box using nothing but the rows and columns that already hold an 8.
Reading the crosshatch
Pick the digit that already appears the most on the board; it has the most rows and columns working for you. For each copy of that digit, trace a line straight across its row and straight down its column. Any empty box that a line passes through loses one candidate cell for that digit. A box that two or three lines pass through often loses every cell but one.
Work box by box across a band (a horizontal strip of three boxes) or a stack (a vertical strip of three boxes). If a digit sits in two of the three boxes in a band, its row lines usually pin down the third box to one or two cells, sometimes to a single one straight away.
Scanning a full band or stack
The technique gets stronger when you compare boxes side by side instead of one at a time. Look at the top band, the three boxes across the top of the grid. If 8 is placed in the left box and the middle box, its two rows rule out two of the three rows in the right box for 8. Only one row is left open there, so 8 can only sit in one of that row's empty cells within the right box. Add a column check and you often land on a single cell without touching a single pencil mark.
This side by side comparison is what separates fast scanning from slow, cell by cell candidate counting. It is also the reason experienced solvers glance at the whole board before committing to one cell.
Common mistakes
- Stopping after one elimination. Ruling out three of five cells feels like progress, but the placement only comes from ruling out all but one. Keep tracing rows and columns until nothing is left to check.
- Forgetting the third unit. A digit is blocked by its row, its column, and its box all at once. Scanning naturally covers rows and columns; do not skip the box itself, since a digit already sitting inside it also rules out cells.
- Scanning a digit that is nearly finished. If a digit already has seven or eight copies placed, cross-hatching for it usually finds one obvious cell fast. If it only has one or two copies, scanning will not narrow much down. Switch digits rather than force it.
When to stop scanning and start marking
Scanning runs out of power once a digit has only one or two placed copies, or once the board fills up enough that most boxes have two or three open cells left rather than five or six. At that point rows and columns stop crossing out enough cells to leave just one, and you need pencil marks to track every candidate for a cell rather than one digit at a time.
Easy and most Medium puzzles on this site can be finished largely by scanning combined with hidden singles. Once you reach Hard boards, scanning still opens the puzzle up, but you will need pencil marks and moves like naked pairs to close it out.
Easy boards have enough givens that scanning alone clears most of the grid. Watch how quickly a digit with several copies gets pinned down.
Frequently asked questions
Scanning, also called cross-hatching, means tracing the rows and columns that already hold a digit to see which cells in a box are still open for it. It is usually the first technique players use, often without a name for it.
Yes, in effect. A hidden single is a digit with only one possible cell left in a row, column or box. Scanning is the manual, eyes-only method most people use to find that cell.
Scanning tracks one digit at a time across the whole board. Pencil marks track every possible digit for one cell at a time. Scanning is faster on open boards; pencil marks become more reliable as the grid fills up.
Cross-hatching needs a digit to already have several copies placed so its rows and columns can rule out enough cells. Hard and Expert puzzles start with fewer givens, so digits take longer to build up that coverage.
No. Start with whichever digit already has the most copies on the board, since it gives you the most rows and columns to trace. There is no fixed order beyond that.
On Easy boards, scanning combined with hidden and naked singles is usually enough to finish. Medium and harder puzzles reach points where scanning cannot narrow a box down to one cell, and you need pencil marks or pair-based techniques instead.