What is Sudoku? A Complete Guide from Beginner to Expert

Understanding the Sudoku Grid

Sudoku is played on a 9x9 grid containing 81 cells. The grid is divided into nine 3x3 boxes, each separated by thicker lines. At the start of every puzzle, some cells are pre-filled with numbers called 'givens.' Your task is to fill every remaining empty cell with a number from 1 to 9.

The grid has nine rows running horizontally and nine columns running vertically. Each cell sits at the intersection of exactly one row, one column, and one 3x3 box. This triple membership is what creates the puzzle's complexity, because every number you place must satisfy all three groups at once.

Typical Sudoku puzzles contain between 17 and 40 givens. The minimum number of clues needed for a unique solution is 17, proven by mathematicians in 2012. Beginner puzzles offer 35 to 40 givens, while expert puzzles might give you only 22 to 25.

The Three Fundamental Rules

Rule one: every row must contain the digits 1 through 9 exactly once. No number can repeat within the same row. Rule two: every column follows the same constraint. Rule three: every 3x3 box must also contain 1 through 9 without repetition.

These three simple rules create a web of constraints that eliminates possibilities and narrows your options. When you place a 5 in a cell, that 5 is ruled out for every other cell in the same row, column, and box. The puzzle is solved when all 81 cells are filled and every group is complete.

A valid Sudoku puzzle has exactly one solution. This means there is never ambiguity: for every empty cell, only one number will ultimately satisfy all three rules. Puzzle creators verify uniqueness before publishing, so you never have to guess between equally valid options.

Key Sudoku Terminology

A 'candidate' is any number that could potentially go in an empty cell based on current information. As you fill in more cells, candidates get eliminated. 'Pencil marks' or 'notes' are small numbers you write in cells to track candidates visually.

A 'naked single' occurs when a cell has only one remaining candidate after you check its row, column, and box. A 'hidden single' is when a number can only go in one cell within a group, even though that cell might have multiple candidates listed.

Other terms you will encounter: 'elimination' means removing a candidate from a cell, 'scanning' means checking rows or columns for placement opportunities, and 'crosshatching' means focusing on one box and checking where each missing number can go.

Your First Steps: How to Start Solving

Begin by scanning the grid for rows, columns, or boxes that are nearly complete. If a row already has eight numbers, the ninth is determined. This gives you an easy first placement and builds momentum.

Next, try crosshatching: pick a number (say, 5) and look at where it already appears on the board. For each box that lacks a 5, check which cells in that box could hold it by eliminating cells whose row or column already has a 5. If only one cell remains, place the 5 there.

When simple scanning stops working, switch to pencil marks. Write all possible candidates in each empty cell. Then look for naked singles (cells with one candidate) and hidden singles (numbers that appear as a candidate in only one cell within a group). These two techniques solve most beginner and intermediate puzzles.

Quick Tips for New Players

Never guess. Every Sudoku is solvable through logic alone. If you feel stuck, you have missed an elimination or a single somewhere. Take a break and come back with fresh eyes. Also, work through one number at a time. Pick the digit that appears most on the board and find its remaining placements.

Use the undo feature freely. Making mistakes is part of learning, and the ability to step back without starting over keeps the experience enjoyable. Start with beginner puzzles to build confidence, then increase difficulty as techniques become natural.