# Sudoku Rules

The goal of Sudoku is to complete the grid by filling in the missing numbers. Every Sudoku puzzle comes with a partially filled grid, and your task is to strategically place the remaining numbers in a way that satisfies the rules of the game. A well-designed Sudoku puzzle will have a unique solution that can be reached through logical deduction and elimination.

### Basic Rules of Sudoku

To play Sudoku effectively, it is crucial to understand and follow the basic rules of the game. Here are the key rules:

#### Rule 1: Every Number Occurs Once in Each Row

In Sudoku, each row must contain all the numbers from 1 to 9, with no repetition. This means that every digit from 1 to 9 should appear once and only once in each row of the 9×9 grid.

#### Rule 2: Every Number Occurs Once in Each Column

Similar to the row rule, each column in Sudoku should also contain all the numbers from 1 to 9 without repetition. Every digit must appear once and only once in each column of the grid.

#### Rule 3: Every Number Occurs Once in Each Subgrid

The Sudoku grid is further divided into nine 3×3 subgrids. Just like rows and columns, each subgrid must contain all the numbers from 1 to 9, ensuring that no digit is repeated within the subgrid.

#### Rule 4: No Repetition within a Cell

Within each cell of the 9×9 grid, there can be only one number. Once you place a number in a cell, no other numbers can be placed in that cell. The challenge lies in determining the correct number to place in each cell based on the existing numbers in the grid.

### Understanding the Grid

To successfully solve a Sudoku puzzle, it is essential to understand the structure of the grid. The grid consists of cells, forming rows, columns, and subgrids. Here’s a brief explanation of each component:

#### Rows

A row in Sudoku refers to a horizontal line of cells, stretching from left to right. There are nine rows in total, numbered from 1 to 9.

#### Columns

Columns, on the other hand, are vertical lines of cells, running from top to bottom. Similar to rows, there are nine columns, labeled from 1 to 9.

#### Subgrids

The Sudoku grid is divided into nine 3×3 subgrids, also known as boxes or regions. Each subgrid contains nine cells and represents a smaller square within the overall grid.

### Filling in the Numbers

The process of solving a Sudoku puzzle involves gradually filling in the missing numbers. You start with a partially filled grid and strategically deduce the correct numbers to place in each cell. Here’s how you can go about it:

**Identify Empty Cells**Scan the grid and identify the empty cells that need to be filled. These cells will be denoted by blank spaces or zeros.**Analyze Rows, Columns, and Subgrids**Examine each row, column, and subgrid to determine which numbers are missing. Look for clues and patterns that can help you narrow down the possibilities.**Apply Logical Deduction**Use logical deduction techniques such as elimination and candidate analysis to deduce the correct numbers for each cell. Start with cells that have limited possibilities and gradually work your way through the grid.**Trial and Error**If you reach a point where logical deduction alone cannot solve the puzzle, you may need to resort to trial and error. Make educated guesses and see if they lead to a valid solution. If not, backtrack and try different options until you find the correct placement.

### The Constraint Principle

The essence of Sudoku lies in its constraint principle. The combination of row, column, and subgrid constraints creates a puzzle that requires careful thinking and strategic decision-making. By adhering to these constraints, you ensure the uniqueness and solvability of each Sudoku puzzle.

### Tips for Solving Sudoku

To improve your Sudoku-solving skills and tackle even the most challenging puzzles, consider the following tips:

**Start with Easy Puzzles**If you’re new to Sudoku, begin with easy puzzles to familiarize yourself with the rules and basic solving techniques. As you gain confidence, gradually move on to more difficult puzzles.**Scan the Grid**Take a quick scan of the grid before making any moves. Look for obvious placements or any patterns that can provide immediate clues.**Focus on One Number at a Time**Instead of trying to fill in all the numbers simultaneously, focus on one number at a time. This approach allows you to concentrate and make more accurate deductions.**Use Pencil Marks**In complex puzzles, it can be helpful to make small pencil marks in the corners of empty cells, indicating the potential numbers that could fit. These marks serve as reminders and aid in the elimination process.**Practice Regularly**Like any skill, Sudoku requires practice to improve. Dedicate regular time to solving puzzles, and challenge yourself with increasing difficulty levels to sharpen your problem-solving abilities.

### Difficulty Mode

On our site you can choose from several difficulty levels: EASY, MEDIUM, HARD, VERY HARD. You can use the hints. Right below the playing field there are ONE STEP and ALL buttons. When you press them, they show one or all numbers respectively.