Sudoku Rules: Complete Guide to Rules, Terms & Grid Layout
Sudoku is a classic logic-based puzzle game that challenges players to fill a 9x9 grid so that every row, column, and 3x3 block contains the numbers 1 through 9 exactly once. This guide covers the official rules, explains common Sudoku terms, and provides tips to help you master the game.
What is the Overall Goal of Sudoku?
The objective of Sudoku is to fill every cell in the 9x9 grid with a number from 1 to 9, ensuring that each number appears only once in every row, column, and 3x3 block (also called a box, region, or nonet). Sudoku puzzles are designed to be solved using logic alone—no guessing is required. Every well-constructed Sudoku puzzle has a single unique solution.
Official Rules of Sudoku
- Grid Structure: The game is played on a 9x9 grid, divided into nine 3x3 blocks.
- Numbers Used: Only the numbers 1 to 9 can be entered into each cell.
- Rows: Each row must contain the numbers 1 to 9, with no repeats.
- Columns: Each column must contain the numbers 1 to 9, with no repeats.
- Blocks: Each 3x3 block (nonet) must contain the numbers 1 to 9, with no repeats.
- Givens: Each puzzle starts with some cells already filled in (called “givens” or “clues”) to guide the player.
- Uniqueness: The puzzle is solved when all cells are filled correctly, and each row, column, and block contains all numbers from 1 to 9 exactly once.
Common Sudoku Terms
| Term | Definition |
|---|---|
| Cell | A single square in the grid (there are 81 in total). |
| Row | A horizontal line of 9 cells. |
| Column | A vertical line of 9 cells. |
| Block/Box/Region/Nonet | A 3x3 section of the grid (there are 9 in total). |
| House | Any row, column, or block (27 total in a standard puzzle). |
| Givens/Clues | The numbers provided at the start of the puzzle. |
| Candidate | A possible number that could go in a cell, based on current logic. |
| Pencil Marks | Small notations in a cell to keep track of possible candidates. |
| Naked Single | A cell with only one possible candidate. |
| Hidden Single | A candidate that appears only once in a row, column, or block. |
| Pair/Triple/Quad | Two, three, or four cells in a house that share the same set of candidates. |
| Locked Candidates | Candidates that are restricted to a single row or column within a block. |
| X-Wing, Swordfish, Jellyfish | Advanced solving techniques based on patterns of candidates. |
| Unique Rectangle | A pattern used to avoid multiple solutions. |
| Bivalue Cell | A cell with exactly two candidates remaining. |
| Chain | A sequence of linked candidates used in advanced solving. |
| BUG (Bivalue Universal Grave) | A situation where only bivalue cells remain, requiring special handling. |
How to Solve a Sudoku Puzzle: Step-by-Step
Scan for Obvious Numbers:
Look for rows, columns, or blocks that are nearly complete and fill in the missing numbers.Use the Process of Elimination:
For each empty cell, eliminate numbers already present in the same row, column, and block.Make Pencil Marks:
Write small numbers in empty cells to keep track of possible candidates.Look for Singles:
- Naked Single: Only one candidate remains in a cell.
- Hidden Single: A candidate appears only once in a house.
Apply Advanced Techniques:
As puzzles get harder, use strategies like naked pairs, hidden pairs, X-Wing, Swordfish, and more. (See our Sudoku Techniques page for details.)
Sudoku Difficulty Levels
| Level | Description |
|---|---|
| Easy | Many clues, can be solved with basic logic and singles. |
| Medium | Fewer clues, may require pairs and simple elimination. |
| Hard | Even fewer clues, requires advanced techniques like X-Wing or Swordfish. |
| Evil | Minimal clues, demands expert strategies and deep logic. |
Frequently Asked Questions
Q: Can a Sudoku puzzle have more than one solution?
A: No. A properly designed Sudoku puzzle has only one unique solution.
Q: Is guessing allowed in Sudoku?
A: No. Every puzzle can be solved logically without guessing.
Q: What if I make a mistake?
A: Use an eraser or the undo feature (online) to correct errors. Double-check your logic before filling in a cell.
Q: What are pencil marks?
A: Pencil marks are small notations in a cell to keep track of possible candidates. They help you visualize options and spot patterns.
More Resources
With this guide, you have all the rules and terminology you need to become a Sudoku expert. Happy solving!