Sudoku Knacker
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Sudoku Solving Techniques

Sudoku is a game of logic, not luck. This guide walks you through every essential technique — from the scanning you'll use in every puzzle to the X-Wing that cracks the hardest grids — each with an illustrated example you can follow cell by cell.

Reading the diagrams: lightly shaded cells mark the key positions of the pattern, and the darker cell is where the deduction lands. Small digits are pencil-mark candidates — some boards show only the ones that matter for the technique.
Beginner

The Fundamentals

Three techniques that solve virtually every easy and medium puzzle on their own.

1. Scanning (Cross-Hatching)

Scanning is the first technique every solver learns — and the one you'll use the most. Pick a single digit and a single 3×3 box, then ask: where can this digit go?

In the diagram, four 5s are already placed. Focus on the top-left box: the 5 in row 1 knocks out the top three cells, the 5 in row 3 knocks out the bottom three, and the 5s in column 2 and column 3 knock out two more. Eight of the nine cells are ruled out — so the one cell left standing, row 2 column 1, must be the 5.

Cross out each row and column that already contains your digit. When only one cell in the box survives, that's your answer — no guessing involved.

2. Naked Single

A naked single is a cell where eight of the nine candidates have already been eliminated by its row, column, and box. Only one digit fits, so you can write it in with complete confidence.

Look at the top-left cell. Row 1 already contains 2, 3, and 5, and column 1 contains 4, 6, 7, 8, and 9. Every digit except 1 is visible from that cell — making 1 the only possibility.

Naked singles are why experienced solvers re-scan after every placement: each new digit can create singles in neighboring rows, columns, and boxes.

3. Hidden Single

A hidden single is sneakier: the cell still has several candidates on paper, but for one specific digit it's the only possible home left in its row, column, or box.

Track the digit 7 across row 2 in the diagram. Columns 3, 4, 5, 7, 8, and 9 each already hold a 7 somewhere, and the 7 in the top-left box covers the first two cells of the row. Every position is ruled out except column 6 — that's a hidden single, even though the cell itself has plenty of other candidates.

Ask one question per digit per unit: "Where can the 7 go in this row?" Hidden singles are everywhere — most easy puzzles are little more than a chain of them.

Intermediate

Locked Candidates & Pairs

Once singles dry up, pairs and intersections keep the puzzle moving.

4. Naked Pair

When two cells in the same row, column, or box have exactly the same two candidates, you've found a naked pair. Those two digits are now locked into those two cells — even though you can't yet say which goes where.

In the diagram, the two shaded cells in the top-left box both have exactly {2, 8} left as candidates. Together they claim the 2 and the 8 for the whole box, so both digits can be erased from every other cell in it — the third cell shown drops from {2, 4, 8} to a single candidate: 4.

You never need to know which cell takes which digit. Knowing the pair "occupies" both digits is enough to eliminate them everywhere else in the unit.

5. Hidden Pair

A hidden pair is the mirror image of a naked pair. Instead of two cells sharing the same candidates, it's two digits that can only land in the same two cells.

Scan row 4 in the diagram for the digits 3 and 6. Column and box constraints rule both digits out of columns 3, 5, 6, and 9, leaving only column 2 and column 8 as possible homes. That locks the pair {3, 6} into those two cells — and every other candidate in them (2, 4, 7, 8) can be erased.

Hidden pairs rarely fill a cell on the spot. Their power is slimming two cells down to exactly two candidates, which then feeds singles and naked pairs.

6. Pointing Pair

Pointing pairs use a box to attack a row or column. If a digit's only candidates inside a box all sit on one line, the digit must go somewhere on that line — so it can be removed from the rest of it.

In the center box of the diagram, the digit 8 can only land in row 5: every other empty cell is blocked by the 8s in column 6, row 2, and row 6. One of those two shaded cells must be the box's 8, which means no other cell in row 5 can be an 8 — so the 8 is erased from the highlighted cell's candidates. (Only the relevant candidates are shown.)

Direction matters: box → line. The pair "points" along the row or column it sits on.

7. Box/Line Reduction

Box/line reduction is the pointing pair flipped around: a line attacks a box. If a digit's only candidates in a row or column all sit inside one box, the digit must be inside that box — and can be removed from the box's other cells.

In the diagram, row 7 already contains the digits 1 through 7, leaving only columns 1 and 2 open — and both cells sit in the bottom-left box. Row 7's 9 is therefore locked into that box, so the 9 can be erased from every other cell in it, like the highlighted cell at row 8, column 2. (Only the relevant candidates are shown.)

Pointing pair: box → line. Box/line reduction: line → box. Together, these two are often called intersection removal.

Advanced

Pattern Hunting

The heavy machinery for expert-level grids.

8. X-Wing

Named after the Star Wars fighter, an X-Wing appears when a digit is restricted to exactly the same two columns in two different rows. The four cells form a rectangle — and like the corners of a picture frame, they pin the digit in place.

In the diagram, the digit 4 in row 2 can only go in columns 3 or 7, and the same is true in row 8. However the 4s actually land, columns 3 and 7 each get their 4 from these two rows — so no other cell in those columns can be a 4. The 4 is eliminated from the highlighted cell's candidates. (Only the relevant candidates are shown.)

X-Wings are worth memorizing because they unlock the hard puzzles that scanning and pairs alone can't crack. The same rectangle logic extends to Swordfish (three rows and three columns).

Put the Techniques to Work

Practice

Play Sudoku Online

Generated puzzles from easy to expert — the perfect training ground for each new technique.

Stuck?

Use the Solver

Enter a stubborn puzzle and get instant hints or the full solution — entirely in your browser.

Offline

Printable Puzzles

Print a batch of puzzles and practice cross-hatching with a good old-fashioned pencil.

Basics

Sudoku Rules

New to the game? Catch up on the rules in a five-minute read.