Reader's Primer

The rules of Sudoku, properly stated

A short, exact guide for anyone picking up the puzzle for the first time — or returning after a long absence.

Sudoku looks like a number game, but it is a logic puzzle in disguise. The digits one through nine are merely placeholders; you could play it with nine letters, nine shapes, nine colours of paint. What matters is the structure underneath. Once you see the structure, the rules collapse into a single sentence: every row, every column, and every box must contain each of the nine symbols exactly once. The rest of this page unpacks what that sentence quietly assumes.

The grid

A Sudoku grid is a 9×9 square — eighty-one cells in total — partitioned in two ways at once. Read by rows it is nine bands of nine cells. Read by columns it is nine ribbons of nine cells. And read as a 3×3 arrangement of 3×3 sub-grids, it is nine boxes. The thicker rules on a printed Sudoku grid mark the boundaries of these boxes; if your eye is ever unsure which cells belong to which region, look for the heavier ink.

Worked example: a solved grid

Pick any row, column, or box. Each contains the digits 1–9 exactly once.

The three rules

Stated plainly, with no flourish:

1. Each of the nine rows contains the digits 1 through 9, each exactly once.
2. Each of the nine columns contains the digits 1 through 9, each exactly once.
3. Each of the nine boxes contains the digits 1 through 9, each exactly once.

That is the entirety of the game. There is no fourth rule. There is no rule against any particular kind of guess, no rule about diagonals, no rule that some digits weigh more than others. Variants exist — killer, jigsaw, hyper — but classic Sudoku is exactly these three constraints, no more.

What a puzzle is, and what a "given" is

A Sudoku puzzle is a partially filled grid: somewhere between 17 and 40 cells have been marked at the outset, and the rest are empty. The pre-filled cells are called givens or clues. A well-formed Sudoku has exactly one solution — there is a single arrangement of the remaining digits that satisfies the three rules. Puzzles printed in newspapers, sold in books, and generated by Very Sudoku all carry this guarantee. If a puzzle has more than one solution it is not a Sudoku; it is a draft.

Givens are conventionally printed in upright type and your own placements in italic, or in a different colour, so the original puzzle is always visible behind your work. On Very Sudoku the givens are set in upright black ink; your entries are set in a slanted India red.

How to play

Pick an empty cell. Ask: which digit can go here without breaking any of the three rules? If you can find exactly one answer, that digit belongs in the cell. If you find none, look elsewhere — the cell will become decidable once something nearby is filled in. If you find more than one, mark them as candidates (often called pencil notes) and move on; you will return when the picture is clearer.

Sudoku has no time limit, no penalty for thinking, and no scoring beyond the satisfaction of a completed grid. The only requirement is patience — and, occasionally, the courage to erase.

Tools the modern grid provides

• Pencil notes. Small candidate digits written into a cell, useful when more than one number is plausible. On Very Sudoku, toggle pencil mode and tap a digit to add or remove it from a cell's notes.
• Auto-check. An optional setting that flags any digit you place that contradicts the solution. Useful for learners; some solvers prefer it off, treating each mistake as a clue in itself.
• Hints. A nudge towards the next deduction. A good hint reveals the easiest logically forced move available on the board, not just a random correct digit.
• Undo and redo. Mistakes are part of the game; the grid forgives.

Where to go next

Once the rules feel comfortable, the next question is not what the puzzle asks but how to answer it efficiently. There is a small vocabulary of logical patterns — each with a name — that competent solvers recognise on sight. Start with the two that carry the most weight: the naked single and the hidden single. Together they will dispatch most easy and medium puzzles without a single guess. See the full techniques index for harder patterns.