Lesson 2 of 9
Cross-hatching
Sweep one digit across the grid and let the rows and columns you already have rule out squares for you.
What this grid does
- Starting digits
- 24 of 81
- Deductions to solve
- 59
- Uses of Cross-hatching
- 25
- Squares placed by scanning
- 57
- First needed at
- deduction 1, in box 2 (R1C4, R1C5, R1C6, R2C4, R2C5, R2C6, R3C4, R3C5 and R3C6, on the 4)
- Also calls for
- last free cell, naked single, hidden single and pointing pairs
Measured on this grid by the same solver the hint button uses — how these puzzles are chosen.
Why this one is worth drilling
Every Sudoku needs this one — it is unavoidable, which makes it the technique you cannot avoid drilling either. That figure is measured rather than guessed: every logically solvable grid measured, at every clue count.
Being stuck here almost always means an incomplete sweep rather than a hard position. If nine digits in a row turn up nothing, one of the nine was skipped — most often in a box you had already looked at and dismissed.
Where it sits in the nine
Second because it is the first technique that asks you to reason about a digit rather than a square, and every later lesson inherits that habit. It is also where most newcomers stop: a solver who cross-hatches well can finish every easy puzzle on this site and will never need lesson four until they try a medium one.
Working it
Pick a digit and finish it everywhere before moving on. Resist jumping to a different digit the moment one looks promising: the discipline of completing a digit is what makes scanning reliable rather than lucky, and it is the only way to be sure a grid has genuinely stalled rather than merely resisted a careless pass. Start with the digits that already appear most often. A digit placed six times constrains the remaining boxes far more sharply than one placed twice, so sweeping it first collapses more of the grid for the same effort. Counting how many of each digit are on the board before you begin takes ten seconds and reorders the whole solve.
The full explanation of cross-hatching covers the reasoning with diagrams.
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