Sudoku
The classic number placement logic puzzle
Sudoku is the world's most recognized logic puzzle, played by millions daily. The rules are elegantly simple: fill a 9x9 grid so that every row, column, and 3x3 box contains the digits 1 through 9 exactly once. No math is required — only pure logical deduction. What makes Sudoku endlessly compelling is how such simple constraints create deep, satisfying chains of reasoning that reward patience and systematic thinking.
History & Origins
Sudoku's roots trace back to 18th-century Swiss mathematician Leonhard Euler and his work on Latin Squares — grids where each symbol appears once per row and column. The modern puzzle as we know it was first published in 1979 by American architect Howard Garns under the name "Number Place" in Dell Magazines. It was then picked up by Japanese publisher Nikoli in 1984, who gave it the name Sudoku (short for "the digits must remain single"). The puzzle exploded internationally in 2004 when Wayne Gould introduced it to The Times of London, sparking a worldwide craze that continues today.
How to Play Sudoku
The Grid
Sudoku uses a 9x9 grid divided into nine 3x3 boxes. Some cells are pre-filled with digits (givens) that anchor your deductions.
Row Rule
Every row must contain each digit from 1 to 9 exactly once. No repeats allowed in any horizontal line.
Column Rule
Every column must also contain each digit from 1 to 9 exactly once. No repeats in any vertical line.
Box Rule
Each of the nine 3x3 boxes must contain digits 1 through 9 with no repeats. Boxes add a spatial constraint layer.
No Guessing
Every well-designed Sudoku has exactly one solution reachable through logic alone. If you need to guess, look for more deductions.
Pencil Marks
Use pencil marks (candidate notation) to track which digits are still possible in each cell. This is essential for harder puzzles.
Strategy & Solving Tips
Mastering Sudoku means building a toolkit of techniques you can apply systematically. Start with scanning and progress to advanced elimination methods as puzzles get harder.
- Start with Naked Singles: if a cell has only one possible digit, fill it in immediately
- Use Hidden Singles: if a digit can only go in one place within a row, column, or box, it must go there
- Apply Pointing Pairs: when candidates in a box are confined to one row or column, eliminate those candidates from the rest of that row or column
- Try Box/Line Reduction: the inverse of pointing pairs — confine candidates and eliminate within the box
- Learn Naked Pairs and Triples: when two cells in a unit share the same two candidates, eliminate those candidates from other cells in that unit
- For expert puzzles, study X-Wing and Swordfish patterns that eliminate candidates across multiple rows and columns simultaneously
Common Mistakes to Avoid
- Forgetting to update pencil marks after placing a digit — stale candidates lead to contradictions
- Guessing when stuck instead of looking for more deductions — a valid Sudoku never requires guessing
- Only scanning rows and columns while neglecting the 3x3 box constraints
- Placing a digit that satisfies the row and column but violates the box rule
- Not using pencil marks at all on medium and hard puzzles — trying to hold candidates in your head leads to errors
Sudoku FAQ
Is Sudoku a math puzzle?
No. Despite using numbers, Sudoku is purely a logic puzzle. You could replace the digits with letters, colors, or symbols and the puzzle would work identically. No arithmetic is involved — only placement logic.
What makes a Sudoku puzzle valid?
A valid Sudoku has exactly one solution that can be reached through logical deduction alone, without guessing. The number and placement of given digits must be sufficient to guarantee a unique answer.
How do difficulty levels work?
Difficulty is determined by which solving techniques are required. Easy puzzles can be solved with basic scanning. Medium puzzles need pencil marks and pairs. Hard and expert puzzles require advanced techniques like X-Wings or chaining.
What should I do when I'm stuck?
First, check your pencil marks are up to date. Then look for Hidden Singles in every row, column, and box. If still stuck, look for Naked Pairs or Pointing Pairs. Taking a break and returning with fresh eyes often helps spot patterns you missed.
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Ready to Play Sudoku?
Challenge yourself with Sudoku — the timeless number placement puzzle that sharpens logical thinking and rewards methodical problem-solving. Multiple difficulty levels ensure there's always a satisfying challenge waiting.