How this walkthrough works
Most sudoku guides list the techniques and leave you to spot them on your own board. This one does the opposite: it takes a single tough-looking grid and solves the opening out loud, naming the technique behind each move as it happens. The point is not to memorise this puzzle — it is to see how a solver decides what to look at next, because that habit is what transfers to every board you meet. When a term appears, we link it to the sudoku glossary rather than redefining it here, so the walkthrough stays about the moves. If you have never placed a digit before, read how to play sudoku first and come back.
The board we will solve
Here is the starting grid. We will refer to cells by row and column: R1 is the top row and R9 the bottom; C1 is the leftmost column and C9 the rightmost. So "R1C6" means the cell in the top row, sixth from the left. The nine three-by-three boxes are numbered 1 to 9, left to right and top to bottom, so box 1 is the top-left block and box 5 is the centre.
| C1 | C2 | C3 | C4 | C5 | C6 | C7 | C8 | C9 | |
|---|---|---|---|---|---|---|---|---|---|
| R1 | 5 | 3 | · | · | 7 | · | · | · | · |
| R2 | 6 | · | · | 1 | 9 | 5 | · | · | · |
| R3 | · | 9 | 8 | · | · | · | · | 6 | · |
| R4 | 8 | · | · | · | 6 | · | · | · | 3 |
| R5 | 4 | · | · | 8 | · | 3 | · | · | 1 |
| R6 | 7 | · | · | · | 2 | · | · | · | 6 |
| R7 | · | 6 | · | · | · | · | 2 | 8 | · |
| R8 | · | · | · | 4 | 1 | 9 | · | · | 5 |
| R9 | · | · | · | · | 8 | · | · | 7 | 9 |
A board this sparse looks intimidating, and that blank feeling is exactly what stops most people. The cure is to stop staring at the whole grid and start interrogating one unit — one row, column, or box — at a time. That is what a unit-by-unit scan does, and it is where every solve begins.
Step 1 — a hidden single
Look at box 2, the top-middle block covering rows 1 to 3 and columns 4 to 6. It already holds the digits 7, 1, 9, and 5, so it still needs 2, 3, 4, 6, and 8. Take just the digit 8 and ask where it can legally sit in this box. Check each empty cell in turn: R1C4 and R3C4 both share column 4 with the 8 already placed at R5C4, so 8 is barred from both. R3C5 shares column 5 with the 8 at R9C5, so it is out. R3C6 shares row 3 with the 8 at R3C3, so it is out too. That leaves exactly one cell in the whole box where an 8 can go: R1C6.
Because 8 has only one possible home in the box — even though R1C6 could, on its own, still take several other digits — this is a hidden single. Place the 8 at R1C6. Hidden singles are the workhorse of Medium and Hard puzzles: you find them by picking one digit and hunting it through a unit, not by waiting for a cell to run out of options.
Step 2 — a naked single
Now train the same discipline on a single cell: R5C5, the very centre of the board. Instead of chasing one digit through a unit, we list everything this one cell can already see. Its row (row 5) contains 4, 8, 3, and 1. Its column (column 5) is dense — it already holds 7, 9, 6, 2, 1, and 8. Its box (box 5) contains 6, 8, 3, and 2. Pool all of those together and you have every digit from 1 to 9 accounted for except one: 5.
With eight of the nine digits blocked, only a 5 can go in R5C5 — a textbook naked single. Place it. Naked and hidden singles are two sides of the same coin: a naked single is a cell with one candidate left; a hidden single is a digit with one cell left. A good solver flips between the two views constantly, and on easier boards those two moves alone will carry you home.
Step 3 — a pointing pair
When the free singles run dry, you stop placing digits for a moment and start removing possibilities. Look again at box 1, the top-left block, and ask where a 7 could go in it. Row 1 already carries a 7 at R1C5, so the box's top cell, R1C3, cannot be the 7. Column 1 already carries a 7 at R6C1, so R3C1 is out as well. That leaves only two candidates for the 7 in this box: R2C2 and R2C3 — and, crucially, both of them lie in the same row, row 2.
Here is the deduction. Wherever the 7 in box 1 finally lands, it is somewhere in row 2. So a 7 can no longer appear anywhere else in row 2, which lets us erase 7 from the candidate lists of R2C7, R2C8, and R2C9 over in box 3. We have not placed a digit, but we have narrowed three cells — and those eliminations are what let the next single appear. This move is a pointing pair: two candidates trapped in one line inside a box, pointing the digit out along that line. Hard puzzles are largely a chain of eliminations like this one, each unlocking the next.
Step 4 — box-line reduction, the mirror move
The box-line reduction is the pointing pair read backwards, and it is worth training your eye to catch both directions. In the pointing pair you looked inside a box and found a digit confined to one line. In the box-line reduction you look along a line and find a digit confined to one box.
Picture working a single column and noticing that a digit — say 4 — has only two surviving candidate cells in that column, and that both of them happen to fall inside the same three-by-three box. The 4 for that whole column is now trapped inside that one box, so it cannot be the answer for any of the box's other cells. You can strike 4 from the rest of that box, even the cells in the box's other two columns. Where the pointing pair clears the rest of a line, the box-line reduction clears the rest of a box. Same logic, opposite direction — and together they are the pair of eliminations that define solid Expert-level solving.
Step 5 — when to reach for an X-Wing
Singles place digits; pointing pairs and box-line reductions trim candidates. When even those dry up and the board simply stalls, you have reached the territory that separates Master and Grandmaster puzzles from the rest, and the first advanced pattern to try is the smallest "fish": the X-Wing.
Here is the shape. Pick a digit — say 3 — and find two rows in which that 3 has been narrowed to exactly two possible cells, and in which those two cells sit in the same pair of columns. The four cells form a rectangle. In each row the 3 must take one of its two cells, and between them they claim one cell in each column — so both columns are now spoken for, and the 3 can be eliminated from every other cell in those two columns. The pattern works identically with rows and columns swapped, and its bigger relatives, the swordfish and jellyfish, extend the same idea across three and four lines.
A candid note on our board: the grid above resolves through singles and locked-candidate eliminations well before it needs a fish — it is a teaching position, not a Grandmaster torture test. But the ladder is the point. On a genuinely hard board the moves are the same in kind, just rarer and better hidden, and the X-Wing is often the move that breaks a long stall. Our difficulty levels guide lays out which techniques each level is built to demand.
Carrying it home
Every hard solve is this same loop: scan a unit, place the singles it hands you, and when they run out, switch to elimination — pointing pairs, box-line reductions, and, at the top levels, a fish — to open the next single. Work only in reasons: if you cannot say why a digit belongs in a cell, it does not belong there yet, because a well-formed puzzle never needs a guess. Every board in Sudoku by WizusLabs is generated on device with exactly one solution, and candidate notes are built in to carry these partial eliminations for you. Free play is ad-supported, with an optional Pro upgrade that removes the ads; the puzzles, modes, and daily challenge are the same either way.
Keep reading: sudoku glossary · classic sudoku techniques · killer sudoku rules and strategy