Sudoku is a web logic game about elimination. The board is 9 rows by 9 columns, 81 squares split into 9 boxes of 3 by 3. The printed givens cannot be changed; you write 1 to 9 into the rest so that all 27 units — 9 rows, 9 columns, 9 boxes — hold each digit exactly once. The library is generated offline and every puzzle passes two solvers: one counts solutions and confirms there is exactly one, the other replays the tier's technique set step by step and confirms both that it finishes and that the tier below gets stuck. That pair is what the four difficulties actually mean. Starter needs only the only-candidate rule, Easy adds the only-place-in-a-box rule, Medium adds box-line exclusion, and Hard adds naked and hidden pairs. A repeat is never blocked: the digit goes in and every square it clashes with is marked alongside it, givens included, and you can carry on. Fill all 81 squares with no repeats and the puzzle is done. Notes write a digit as small corner text, undo steps back move by move, and the hint works in four levels: it lights a box, then the square, then names the technique, and only then writes the digit in.

The last ten minutes of a class, the grid in the newspaper corner gone over so many times the pencil had scuffed the paper. Another one drawn by hand on the back of an exercise book, the lines ruled slightly crooked and filled in anyway. What held your attention was never the difficulty — it was the certainty of a square that could only be one thing. No guessing; one step, then the next.

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1. 01

   Tap the board once to start. One square is outlined at the opening, and on the Starter and Easy tiers that square can always be deduced on the spot — it is the cheapest place to begin.

2. 02

   Tap a square to select it, then tap a digit on the strip below to write it in. Tap the digit that is already there and it gets erased.

3. 03

   With a square selected, its row, column and box dim-light and every matching digit on the board lights up. Once a digit has been placed 9 times its key greys out and a line is struck through it.

4. 04

   When you are unsure press Notes: a digit then lands as small corner text instead. Press it again to go back to writing. Erase clears a square's digit and its corner notes together.

5. 05

   If you are stuck press Hint. It works a level at a time: first the box, then the square, then the name of the technique to use, and only on the fourth press does it write the digit in.

6. 06

   On a keyboard the arrow keys move the cursor, 1 to 9 write and hop to the next empty square, 0 or Backspace erases, N toggles notes, U undoes, H hints, and Esc or P pauses.

Sweep the boxes first: the busiest box has the fewest places left, so settling what is certain there loosens the whole grid. Two placement rules carry the first two tiers: a square with one digit left takes it, and a digit with one place left in a box goes there. Tap a filled square and every copy of that digit already on the board lights up. Scanning those lit squares for the box that still has an open place is far faster than reading square by square. From Medium onward you need box-line exclusion: if all the places for a digit inside one box sit on the same row, that digit is impossible elsewhere on that row, and the mirror statement holds for a row confined to one box. Hard needs pairs: two squares in a unit whose candidates are the same two digits own both of them, so the rest of the unit can drop them. Two digits whose places in a unit are the same two squares do the same thing from the other side. When unsure, note rather than write. A note creates no clash mark and does not use up a digit key early, which makes it a safer place to park a guess than your memory.

How do you play sudoku?

The board is 9 rows by 9 columns, 81 squares divided into 9 boxes of 3 by 3. Write the digits 1 to 9 into the empty squares so that no row, column or box repeats a digit. The printed givens cannot be changed, and filling all 81 squares with no repeats completes the puzzle.

Where should you start a sudoku?

Start with the busiest row, column or box, because that is where the fewest options remain. This version outlines one square at the opening, and on the Starter and Easy tiers that square can always be deduced with the tier's own technique. Selecting any filled square also lights every copy of that digit, which beats reading square by square.

Does sudoku ever require guessing?

Not here. Every puzzle is verified to have exactly one solution and to be solvable step by step inside its tier's technique set: only-candidate on Starter, then only-place-in-a-box, then box-line exclusion, then naked and hidden pairs. Puzzles that would need deeper chain reasoning are rejected while the library is being generated.

What happens if you write a wrong digit in sudoku?

Nothing is blocked and nothing is deducted. If the digit now repeats in its row, column or box, every square in that unit holding it is marked, givens included, and you can keep going. If the board ends up full with repeats still on it, the summary tells you how many squares clash and one tap takes you back to the same board to fix them.

How do notes and undo work in sudoku?

Press Notes and a tapped digit becomes small corner text; tap it again to remove it. Notes only go into empty squares, and Erase clears a square's digit together with its notes. Undo steps back move by move for up to 2048 remembered moves, so writing, clearing, noting and erasing are all reversible.

What sudoku techniques should you learn first?

Learn the two placement rules first: a square with only one possible digit takes it, and a digit with only one possible place in a box goes there. Then learn two elimination rules: when a digit's places inside a box all share a row or column you can clear that digit from the rest of that line, and when two squares in a unit share the same two candidates — or two digits share the same two squares — the candidates tighten. The four tiers here rise in exactly that order.

Which sudoku difficulty should you pick, and how many givens does each have?

Fewer givens does not simply mean harder here: the ranges overlap and a puzzle is tiered by the techniques it takes to solve. The tiers are Starter, Easy, Medium and Hard, with 36 to 45, 30 to 38, 26 to 34 and 22 to 30 givens respectively, and each is named after the hardest technique it requires — only-candidate, then only-place-in-a-box, then box-line exclusion, then naked and hidden pairs. Switching tier opens a new puzzle and drops the current one.

How does the daily sudoku work?

The daily puzzle is the same board for everyone, generated from the UTC date, and it is fixed to the Easy tier. It is only an entry point: once opened it behaves like any other run, with no limit and no expiry. The entry shows when you have finished today's, and a new one appears the next day.