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Sudoku Rules Explained
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Sudoku Rules

The Simple Constraints That Make It All Work

By Minimal Sudoku TeamLast updated:

Sudoku has remarkably simple rules — just three constraints that apply to every puzzle. Understanding these rules deeply is the foundation of all solving techniques.

Quick Summary
  • Grid: 9×9 cells, divided into nine 3×3 boxes
  • Goal: Fill every cell with a digit from 1 to 9
  • Constraint: Each row, column, and box contains 1-9 exactly once
  • No math: It's pure logic — no arithmetic required

The Grid

A standard Sudoku grid is a 9×9 square, giving you 81 cells total. The grid is further divided into nine 3×3 regions called boxes (also known as "blocks" or "regions").

┌───────┬───────┬───────┐
│ Box 1 │ Box 2 │ Box 3 │
│       │       │       │
├───────┼───────┼───────┤
│ Box 4 │ Box 5 │ Box 6 │
│       │       │       │
├───────┼───────┼───────┤
│ Box 7 │ Box 8 │ Box 9 │
│       │       │       │
└───────┴───────┴───────┘

The Three Rules

Sudoku has exactly three rules. Every technique, from the simplest to the most advanced, is derived from these constraints:

Rule 1: Rows

↔️ Row Rule
Each row must contain the digits 1 through 9 exactly once. No digit can repeat within a row.

✓ Valid row: 1, 2, 3, 4, 5, 6, 7, 8, 9

✗ Invalid row: 1, 2, 3, 4, 5, 6, 7, 8, 8 (8 repeats)

Rule 2: Columns

↕️ Column Rule
Each column must contain the digits 1 through 9 exactly once. No digit can repeat within a column.

Rule 3: Boxes

⬜ Box Rule
Each 3×3 box must contain the digits 1 through 9 exactly once. No digit can repeat within a box.
That's It!
These three rules are the entire game. Every solving technique — from Naked Singles to XY-Chains — is just a clever way to apply these constraints.

Given Cells (Clues)

Every Sudoku puzzle starts with some cells already filled in. These are called givens or clues. The difficulty of a puzzle depends on:

Minimum Clues
A valid Sudoku needs at least 17 clues to have a unique solution. Fewer than 17, and there will always be multiple valid solutions.

One Solution

A properly constructed Sudoku puzzle has exactly one solution. This isn't technically a "rule" you use to solve, but it's a property of valid puzzles.

Some advanced techniques (like Unique Rectangle) actually use this property — if a move would create multiple solutions, that move must be wrong.

Getting Started

Now that you know the rules, learn your first solving techniques:

Beginner Techniques

📍Last Remaining Cell1️⃣Naked Single🔎Hidden Single👀Scanning

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