# Sudoku rules (tutorial 2)

## Sudoku Rule 1: Fill in the Grid

The most basic rule of Sudoku is that every cell in the 9x9 grid must contain a number from 1 to 9. At the start of a Sudoku puzzle, you'll see that some numbers are already filled in. Your job is to fill in the remaining numbers.

Example: Imagine a 9x9 grid where all the cells are empty except for one, which contains a "5". You would need to fill in the remaining 80 cells so that each contains a number from 1 to 9.

## Sudoku Rule 2: Each Row Must Contain Each Number Once

Each row of the 9x9 grid must contain each number from 1 to 9 exactly once. No number can be repeated in a row.

Example: If a row in your Sudoku puzzle contains the numbers "1, 2, 3, 4, 5, 6, 7, 8", the only number left to fill in the last cell of that row would be "9". Conversely, if "9" is already present in a row, you can't place a "9" anywhere else in that row.

## Sudoku Rule 3: Each Column Must Contain Each Number Once

Just like with the rows, each column of the 9x9 grid must also contain each number from 1 to 9 exactly once. No number can be repeated in a column.

Example: If a column in your Sudoku puzzle contains the numbers "1, 2, 3, 4, 5, 6, 7, 8", the only number left to fill in the last cell of that column would be "9". Conversely, if "9" is already present in a column, you can't place a "9" anywhere else in that column.

## Sudoku Rule 4: Each Box Must Contain Each Number Once

The 9x9 grid is further divided into nine 3x3 boxes. Each of these boxes must also contain each number from 1 to 9 exactly once.

Example: If a 3x3 box in your Sudoku puzzle contains the numbers "1, 2, 3, 4, 5, 6, 7, 8", the only number left to fill in the last cell of that box would be "9". Conversely, if "9" is already present in a 3x3 box, you can't place a "9" anywhere else in that box.

Remember, the key to Sudoku is understanding that every number from 1 to 9 must appear once, and only once, in each row, column, and 3x3 box. The challenge is figuring out where each number belongs based on the numbers that are already filled in.