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 to go. It is the most used technique in all of Sudoku and the one that fills most of an easy or medium board without any notes at all. Good scanning is less about cleverness and more about a steady, orderly habit. You are not looking for anything hidden; you are simply asking, for one value at a time, where it is allowed to sit. Do that patiently across the grid and forced placements reveal themselves one after another, often faster than you expect, and each one you place narrows the choices for everything around it.

Crosshatching, step by step

Crosshatching is a focused form of scanning aimed at a single box. You pick one value and check whether the rows and columns passing through that box already contain it somewhere else. Each row or column that already holds the value blocks the cells it passes through inside the box. When only one unblocked cell is left, the value must go there, because it has nowhere else it can legally sit.

1. Choose a value that already appears several times across 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 every crossing column that already has the value.
5. If a single cell survives, place the value there with confidence.

A worked example

Say you are placing the triangle in the top-left box, the nine cells from r1c1 to r3c3. You notice a triangle already sits at r1c8, so the whole of row 1 is blocked and r1c1, r1c2, and r1c3 are out. Another triangle at r3c6 blocks row 3, removing r3c1, r3c2, and r3c3. That leaves only row 2 inside the box. Now bring in the columns: a triangle at r7c2 blocks r2c2, and a triangle at r6c3 blocks r2c3. Only r2c1 remains unblocked, so the triangle must go at r2c1. That is a complete crosshatch, and no notes were needed to prove it.

Start with common values, in a fixed order

A value that already appears five or six times on the board is the easiest to place, because it blocks so many cells that a box is often left with a single option. Clearing out the common values first fills the board quickly and creates crowded regions where even more forced moves appear, since each shape you place is a fresh blocker for the next scan. Just as important is the order in which you work. Randomly darting around the board makes you miss easy moves and repeat work you have already done. Pick a routine and stick to it: many solvers go value by value, from the first shape to the last, crosshatching every box before moving on. In Shapedoku the nine shapes give you a natural sequence to follow, so you never lose your place.

When scanning stops working

On harder boards, pure scanning will eventually stall, and that is normal. It is your cue to switch on Notes and look for singles, then pairs and the more advanced patterns. But even then you will return to scanning constantly, because every value you place opens fresh crosshatching opportunities elsewhere. Think of scanning as the engine you keep coming back to rather than a stage you leave behind. Master it first and everything else gets easier, since the advanced techniques mostly exist to create one more placement that your scanning can then build on.

Put it into practice

The fastest way to sharpen this skill is deliberate repetition. Load a Medium board in Shapedoku, pick one shape, and crosshatch every box for it before moving to the next shape. Resist the urge to jump around chasing whatever catches your eye. You will be surprised how much of the puzzle falls into place from this single, orderly pass, and how quickly your eye starts to see the blocked cells without consciously tracing each row and column. That automatic recognition is exactly what separates a slow, hesitant solve from a smooth, confident one.

A common scanning mistake

One mistake slows down almost every beginner: placing a value the moment a cell looks likely, without finishing the check. Scanning is only trustworthy when it is complete, which means confirming that every other cell in the box is genuinely blocked before you commit. A half-finished crosshatch, where you stop at the first plausible cell, is how wrong placements creep in and quietly poison the rest of the grid. Slow down just enough to rule out the alternatives, not only to confirm the one you want. The good news is that a full check barely costs any time once the habit is set, because your eye learns to sweep the crossing rows and columns in a single pass. Certainty is worth those extra seconds; a single guessed placement can cost you many minutes of untangling later.