Sudoku Rules

What Is Sudoku?

Sudoku is a logic-based number placement puzzle played on a 9×9 grid. The goal is to fill every empty cell with a digit from 1 to 9 so that each digit appears exactly once in every row, every column, and every 3×3 box. Despite using numbers, Sudoku requires no arithmetic whatsoever — it is purely a game of logical deduction.

The puzzle originated in the United States in 1979 under the name "Number Place," gained enormous popularity in Japan throughout the 1980s, and became a worldwide phenomenon around 2005. Today it is one of the most widely played puzzle games on the planet, appearing in newspapers, books, apps, and dedicated websites.

The Grid Structure

A standard Sudoku grid contains 81 cells arranged in 9 rows and 9 columns. The grid is also divided into nine non-overlapping 3×3 regions called boxes (sometimes called blocks or regions). Every cell belongs to exactly one row, one column, and one box — three units simultaneously.

• Row: A horizontal line of 9 cells. There are 9 rows in total.
• Column: A vertical line of 9 cells. There are 9 columns in total.
• Box: A 3×3 square region. There are 9 boxes in total, arranged in a 3×3 pattern.

When you place a digit, you must check all three units — the row, column, and box — to make sure that digit is not already present.

The Core Rule

The single rule of Sudoku is: every row, every column, and every 3×3 box must contain the digits 1 through 9 exactly once.

• No digit may appear more than once in any row.
• No digit may appear more than once in any column.
• No digit may appear more than once in any 3×3 box.

These three constraints work together. A cell that sits at the intersection of a row, a column, and a box is restricted by all three simultaneously, which is what makes Sudoku solvable through logical elimination.

The Unique Solution Principle

A well-crafted Sudoku puzzle has exactly one valid solution. This is called the unique solution principle. If a puzzle had multiple solutions, there would be no logical way to determine which path is correct, making it impossible to solve by pure reasoning.

This principle guarantees that you never need to guess. Whenever you feel stuck, it means there is a logical deduction you have not yet found — not that guessing is required. Scanning other rows, columns, or boxes will eventually reveal the next move.

Difficulty Levels

The difficulty of a Sudoku puzzle is determined by how many starting digits (called "givens") are provided and how they are arranged. More givens generally means an easier puzzle; fewer givens means harder.

• Easy: Many givens are provided. Most cells can be filled through straightforward scanning.
• Medium: Requires a few steps of logical chaining before each cell is determined.
• Hard: Advanced techniques such as hidden pairs or X-wing patterns may be necessary.
• Expert: Designed for experienced solvers who are comfortable with complex multi-step reasoning.

Tips for Getting Started

If this is your first time playing, start with Easy difficulty and focus on understanding how the three constraints interact. Look for rows, columns, or boxes that already have many digits filled in — these areas have the fewest remaining possibilities and are the easiest places to find your next move.

Use the pencil mark (candidate) feature to write small potential digits inside empty cells. As you fill in confirmed digits elsewhere, cross out the corresponding candidates. This systematic approach makes the puzzle much more manageable, especially at Medium difficulty and above.