Crosshatching

Crosshatching (or cross-hatching) is the very first thing anyone learns to do in Sudoku — often before they even know it has a name. You take one digit, look at where it already sits, and use those rows and columns to rule it out of a box until only one cell is left. It needs no pencil marks and it's how most of an Easy or Medium grid gets solved.

How crosshatching works

Pick a digit — say 7 — and a box. Any row that already contains a 7 blocks the whole of that row inside the box; the same goes for columns. Cross out every blocked cell, and if a single empty cell survives, that's where the 7 must go. It's called crosshatching because you're mentally drawing lines across from the existing digits.

Step by step

1. Choose a digit that already appears several times on the board — more copies means more lines to work with.
2. Pick a box that doesn't yet contain it.
3. Rule out rows and columns. Any row or column through the box that already holds the digit blocks its cells in the box.
4. Count what's left. One surviving empty cell → place the digit. Two or more → move on and come back later.
5. Repeat for the next box, then the next digit.

Crosshatching and hidden singles

Crosshatching is really the visual way to find a hidden single — a digit that has only one legal home in a unit. As grids get harder the survivors dry up and you'll switch to full pencil marks and techniques like the naked pair. But crosshatching never stops being useful: it's always the fastest way to open a fresh grid.

Common mistakes

• Giving up on a box too soon. If two cells survive for one digit, another digit's crosshatch may remove one of them — come back after a lap.
• Only scanning rows. Columns block just as well; use both.
• Chasing rare digits. Start with the numbers that appear most — they give you the most lines to crosshatch with.

Which Sudoku types is this best for?

The same logic applies across variants, but it pays off more in some than others.