Sudoku: rules, strategy, and free play
Sudoku is the canonical Latin-square logic puzzle. Fill a 9×9 grid so that every row, every column, and every 3×3 box contains the digits 1 through 9 exactly once. Some cells are pre-filled as clues; you deduce the rest. No guessing — a legitimate Sudoku has exactly one valid solution reachable by logic alone.
THE RULES
- Every row contains 1–9 exactly once. No row may repeat a digit or leave one out.
- Every column contains 1–9 exactly once. Same constraint applied vertically.
- Every 3×3 box contains 1–9 exactly once. The grid is divided into nine 3×3 boxes by the bold lines. Each box must also contain every digit 1–9 once — the constraint that makes Sudoku harder than a plain Latin square.
BEGINNER STRATEGY
- Scan for naked singles. If a cell can only legally hold one digit (because the other 8 are already placed in its row, column, or box), place it. This is the easiest deduction and is enough to solve most Easy / Medium puzzles end-to-end.
- Look for hidden singles. Within a row, column, or box, check whether any digit can only go in one cell. The cell may still have multiple candidates overall, but for THIS digit there's only one slot. Place it.
- Use pointing pairs. If all candidates for a digit in a 3×3 box line up in the same row or column, that digit can be eliminated from the rest of that row or column outside the box. A small but powerful constraint.
- Use pencil marks for harder puzzles. On Hard / Expert puzzles, pencil-mark every cell with its candidate digits. Look for naked pairs — two cells in the same unit sharing exactly the same two candidates — and eliminate those two digits from every other cell in that unit.
- Never guess. Sudoku has a unique solution by deduction. If you're guessing, you've missed a constraint. Back up, scan rows / columns / boxes again, and look for whichever logical step you skipped.
COMMON MISTAKES
- Forgetting the box constraint. Most beginners scan rows and columns but overlook the 3×3 box. A digit already placed in the same box eliminates it from a cell just as definitively as a same-row or same-column clash.
- Guessing when stuck. If you feel like you have to guess, you haven't found the right constraint yet. A legitimate Sudoku is fully solvable by logic. Step back, scan a different unit, or start pencil-marking.
- Placing digits instead of eliminating them. The effective Sudoku mindset is elimination, not selection. Don't ask 'which digit goes here?' — ask 'which digits can't go here, and why?' When eight digits are ruled out, the ninth places itself.
- Stopping after one naked single. Placing a naked single often unlocks another naked single in the same row, column, or box. Always re-scan the affected units after a placement — a chain of easy deductions is common.
HOW TO THINK ABOUT IT
Think of each empty cell as having a constraint budget, not a blank slate. Every row, column, and box it belongs to is already "spending" that budget by occupying digits. Your job is to find which cell's budget is spent down to one — then place it. The puzzle doesn't reveal one answer at a time; it reveals one constraint at a time, and the answers follow.
WHY THIS PUZZLE REWARDS YOU
Sudoku is the puzzle most newcomers start with — and the one most veterans return to for a quick deductive workout. The 9×9 size is small enough to solve in 5–20 minutes, the rules fit in three sentences, and the difficulty range scales from gentle warm-up to multi-pencil-mark expert without changing the rules. Players who enjoy Sudoku usually enjoy Killer Sudoku (adds cage sums), Calcudoku (replaces 3×3 boxes with arithmetic cages), and Skyscraper (Latin-square base with visibility clues).
VARIANTS
- Killer Sudoku. The 9×9 grid with standard Sudoku rules — but all clue cells are replaced by dashed cages. Each cage shows a target sum; the digits inside must add up to that target without repeating. No given cells at all.
- Calcudoku (KenKen). Replaces the 3×3 boxes with arithmetic cages using +, −, ×, and ÷. Available in multiple grid sizes (4×4 through 9×9). No box constraint — just row/column Latin square plus cage arithmetic.
- Skyscraper. A Latin-square base (like Sudoku without the box rule) with visibility clues around the perimeter. Each edge number tells you how many 'buildings' are visible from that direction — taller heights block shorter ones behind them.
- Samurai Sudoku. Five overlapping 9×9 Sudoku grids sharing corner boxes. The shared cells must satisfy both grids simultaneously. Significantly harder — typical solve time is 45–90 minutes.
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Same grid, same three rules — the real question is which fits where you are as a solver.
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PLAY IT IN GRIDJOY — FREE ON ANDROID
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