Sudoku: How to Play, Rules and Solving Techniques

The History of Sudoku

Sudoku looks like a Japanese invention, but the modern puzzle was most likely designed by Howard Garns, a retired architect from Indiana. Dell Magazines published it in 1979 under the name Number Place. Earlier French newspaper puzzles in the 1890s used similar grids, though they were not the same game.

In April 1984 the Japanese company Nikoli, led by Maki Kaji, began publishing the puzzle. In 1986 the name was shortened to Sudoku, built from the Japanese words for number and single.

The worldwide boom came later. Wayne Gould, a retired judge from Hong Kong, wrote a program to generate puzzles and offered them to newspapers. The Times of London printed its first Su Doku in November 2004, and other papers and puzzle books soon followed.

How to Play Sudoku: Basic Rules

The standard puzzle is a 9x9 grid divided into nine 3x3 boxes. Some cells already contain digits, called givens or clues. Your job is to fill every empty cell with a digit from 1 to 9. No arithmetic is involved, so the digits are just nine different symbols.

A good puzzle has exactly one solution. That matters: it means every step can be deduced by logic, and you never have to guess. A puzzle needs at least 17 givens to have a single solution, so anything with fewer clues is ambiguous.

1. Look at the grid: each row, each column and each 3x3 box must end up containing the digits 1 to 9 exactly once.
2. Pick a cell and list which digits are still possible there by checking its row, column and box. These possibilities are called candidates.
3. If only one candidate fits a cell, write it in. If a digit fits only one cell in a row, column or box, it goes there.
4. Every new digit removes candidates from its row, column and box, so keep updating your notes after each placement.
5. When simple moves run out, use the techniques below to eliminate candidates until another digit is forced.
6. You win when all 81 cells are filled and no row, column or box repeats a digit.

Sudoku Variants and Difficulty Levels

Most sites label puzzles Easy, Medium, Hard and Expert, but there is no universal scale. Difficulty usually depends on which techniques you need, not just on how many givens appear. A puzzle with 36 clues can be harder than one with 26 if its clues are poorly placed.

Easy puzzles need only single candidates; harder ones require pairs, locked candidates or X-Wing. Beyond the classic grid there are smaller 4x4 and 6x6 versions for beginners, and rule variants such as Killer Sudoku, Jigsaw Sudoku and Diagonal Sudoku.

Sudoku Strategy: Solving Techniques That Work

Strong solvers rely on a short toolkit applied in a steady order. Start with the simplest technique and move up only when it stops producing progress.

• Scan before you note: look for rows, columns and boxes that are nearly full, because they have the fewest options and give quick placements.
• Naked single: when a cell has only one possible digit left, it must be that digit, since nothing else can legally go there.
• Hidden single: if a digit can go in only one cell of a row, column or box, place it there, even if the cell has other candidates.
• Naked pair: if two cells in one group share the same two candidates, those digits belong to those cells, so remove them from every other cell in the group.
• Locked candidates (pointing pairs): if a digit in a box can only appear in one row or column, remove it from the rest of that line outside the box.
• X-Wing: if a digit has just two possible places in each of two rows, and they line up in the same two columns, remove it from the rest of those columns.
• Never guess: a valid puzzle always has a logical path, so if you feel stuck, re-check your candidates for a missed elimination before trying anything else.

Frequently Asked Questions

What are the basic rules of Sudoku?

Fill a 9x9 grid so that every row, every column and every 3x3 box contains each digit from 1 to 9 exactly once. Some cells are pre-filled as clues. You never add numbers together, so Sudoku is a pure logic puzzle rather than a maths exercise.

Who invented Sudoku?

The modern puzzle was most likely created by Howard Garns, an American architect, and published by Dell Magazines in 1979 as Number Place. It was popularized in Japan by Nikoli, which gave it the name Sudoku in 1986, and later became a worldwide hit through newspapers.

Does every Sudoku puzzle have only one solution?

A well-made puzzle does. Having a single solution means each step can be found by logic, with no guessing. Poorly made or random grids can have several solutions, and mathematicians have shown that a puzzle with fewer than 17 clues cannot have a unique answer.

How do I get better at Sudoku?

Learn techniques one at a time. Master naked and hidden singles first, since they solve most easy puzzles. Then add naked pairs and locked candidates. Writing small candidate notes in each cell helps, and practicing regularly makes patterns easier to spot.

What makes a Sudoku puzzle hard?

Difficulty comes mostly from the techniques required, not from the number of clues. A hard puzzle may need pairs, locked candidates or X-Wing patterns before any digit can be placed. Fewer givens usually make a puzzle harder, but clue placement matters more.

Will there be a free online Sudoku on PureTable Games?

Yes, a free browser version is planned, with no sign-up needed and puzzles at different difficulty levels. It is not live yet, so this page currently explains the rules, history and strategy. Check back later for the launch.