Scanning is looking at the grid as it stands and asking where a particular digit can go. It needs no notes, no bookkeeping and no memory, which is why it should always come first. On an easy puzzle it can finish the whole thing.
Cross-hatching, step by step
Take one digit — start with whichever appears most often in the clues. Find a box that does not contain it. Now trace the rows and columns: any row containing that digit rules out its whole row inside the box, and the same for columns. If exactly one cell in the box survives, you have a placement.
The name comes from the mental picture: lines coming in from the sides and top, crossing out cells until one is left.
Work digit by digit, not cell by cell
This is the single change that makes scanning fast. Beginners look at an empty cell and wonder what goes in it, which requires checking nine digits against three regions. Instead, pick a digit and sweep all nine boxes with it. You are then checking one thing at a time, and the eliminations from each box carry over to the next.
Finish a digit completely before moving on. When a digit is placed all nine times, mentally retire it — the number pad here dims digits that are fully used, so you can see at a glance which ones are still live.
Choosing the order of digits
Start with the digit that has the most clues on the board, because each existing instance blocks a row and a column. A digit appearing six times constrains the grid far more than one appearing twice. Work down from the most common to the least.
The five-or-more rule
A digit already present five or more times will usually place at least one more instance by scanning alone. Sweep those digits first and you will get placements before you have done any real thinking.
Scanning rows and columns, not just boxes
Most people scan boxes and stop there. The same logic works on a row: if a row is missing only a 2 and a 9, and one of the two empty cells sits in a column that already has a 9, then that cell is the 2 and the other is the 9.
Rows and columns with many clues are worth checking directly for exactly this reason. Count the gaps; a row with two gaps is often resolvable in one look.
When to stop scanning
Do a full sweep of all nine digits. If that sweep produces no placements at all, scanning is exhausted for now and it is time for candidates. If it produces even one, do another full sweep — each placement changes the constraints and can unlock several more.
That loop is the whole method on easy and most medium puzzles: sweep, place, sweep again, until a sweep comes back empty.
Speeding it up
- Use the highlight. Selecting a cell containing a 7 shows every other 7 on the grid, which makes the blocked rows and columns obvious.
- Look at the most crowded box first. A box with seven clues has only two possibilities left and often resolves instantly.
- Scan three boxes at a time — the horizontal band or vertical stack. Digits interact strongly within a band, and two placements in a band frequently force the third.
The band technique
Take the three boxes across the top. If a digit appears in two of them, the third box must contain it in the one row that is not already used. That often narrows it to two or three cells, and a single column clue then finishes the job. This is the most productive scanning pattern on medium puzzles and it is easy to miss if you only ever look at one box at a time.