Guide · Beginner
Scanning: The Fastest Way to Start Any Sudoku
Before pencil marks, before pairs and wings, there is scanning. It is the first technique every solver learns, and even experts use it to open every puzzle.
What scanning means
Scanning means looking along rows and columns to see where a number cannot go. When all but one cell is ruled out, you have found where the number must go. Solvers also call it cross-hatching, because you imagine lines crossing the grid.
Step by step
- Choose a number, for example 7.
- Find every 7 already in the grid.
- From each 7, imagine a line across its row and down its column. No other 7 can sit on those lines.
- Now look at a box that has no 7 yet. Count the empty cells in that box that are not on any of your imaginary lines.
- If exactly one cell is left, write 7 there.
Scanning a row or column
Scanning works on rows and columns too. Take a row with only two or three empty cells. Which numbers are missing? For each missing number, check the columns of the empty cells. If a missing number already appears in one of those columns, it cannot go in that cell. Often only one choice remains.
Slicing and dicing
Look at a band of three boxes side by side. If the number 4 appears in two of those boxes, in two different rows, the third box must hold its 4 in the remaining row. You have just narrowed the search to three cells, sometimes one. Solvers call this slicing and dicing, and it is just scanning applied to a whole band at once.
A good scanning routine
- Go through the numbers 1 to 9 in order, once.
- Every time you place a number, check the boxes around it again. One new number often unlocks another.
- When a full pass through 1 to 9 finds nothing new, it is time for pencil marks.
Why it works
Every box needs each number once. Scanning simply removes the cells where a number is forbidden. It never guesses, so every number you place by scanning is certain.
With practice, your eyes will start doing this automatically. That is the moment Sudoku starts to feel fast and smooth.