Scanning and Crosshatching: The First Skills to Master

What scanning is

Scanning means sweeping your eyes along rows, columns and boxes to find where a value is forced. It is the most used technique in Sudoku and the one that fills most of an easy or medium board. Good scanning is less about cleverness and more about a steady, orderly habit.

Crosshatching, step by step

Crosshatching is a focused form of scanning for a single box. You pick one value and check whether the rows and columns passing through a box already contain it. Each row or column that holds the value blocks the cells it passes through. When only one cell in the box is left unblocked, the value must go there.

1. Choose a value that already appears several times on the board.
2. Look at one box that does not yet contain that value.
3. For each row crossing the box that already has the value, rule out the cells in that row.
4. Do the same for each crossing column.
5. If a single cell remains, place the value there.

Why start with common values

A value that already appears five or six times on the board is easy to place, because it blocks many cells and often leaves just one option per box. Clearing out the common values first fills the board quickly and creates the crowded areas where even more moves appear.

Scan in a fixed order

Randomly darting around the board causes you to miss easy moves and repeat work. Instead, pick a routine and stick to it. Many solvers go value by value, from the first shape to the last, crosshatching each box in turn. A consistent order means you never overlook a forced placement.

When scanning stops working

On harder boards, scanning alone will eventually stall. That is your cue to switch on notes and look for singles, then pairs. But even then you will return to scanning constantly, because every value you place opens up fresh crosshatching opportunities. Master this first, and everything else gets easier.

Put it into practice on a Medium board in Shapedoku. Pick one shape, crosshatch every box, then move to the next shape. You will be surprised how much of the puzzle falls into place.