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What is Sudoku?

Sudoku (数独, sūdoku), is a logic-based placement puzzle to enter the digits 1 through 9 in each cell of a 9×9 grid.
Sudoku puzzle was based on the Latin squares, a puzzle devised by a Swiss mathematician Leonhard Euler, in 1783. The first puzzle was invented by Howard Garns in 1979 and published in an American magazine “Dell Magazine” with the name “Number Place”. However, it became popular in Japan when published by Nikoli Publishing in 1984. The name of “Sudoku” (数独) was from the Japanese abbreviation of a longer phrase, "Suji wa dokushin ni kagiru" , means that "the numbers must occur only once". Nikoli also introduced the rule that given numbers in puzzles should be placed in a symmetrical pattern.

What are the rules?


The game simply requires that the grid is completed with different numbers in every row, column and box. The existing numbers cannot be changed or moved. In a correctly formed Sudoku puzzle, there is one unique solution. Sudoku does not require any knowledge of mathematics to solve. However, Sudoku does require logic and reasoning.

For a 9×9 grid to be a valid Sudoku grid, the following 3 requirements should be fulfilled:
1. for each row: every number from 1 to 9 should occur exactly once
2. for each column: every number from 1 to 9 should occur exactly once
3. for each 3×3 square with a thicker border (there are 9 of them): every number from 1 to 9 should occur exactly once

The terminology


The smallest unit in the 9×9 grid known as "Cell". Sometimes a cell also known as "square".

A "Column" made up of 9 vertical cells show in BLUE colour.

A "Row" made up of 9 horizontal cells show in RED colour.

A "Block" made up of 3×3 sub grids. (as known as “regions”),



The 'buddy' of a square is the squares that share a row, column or 3x3 blocks.

e.g. the buddies of red circle Row7 Column3 (R7C3) are shown below. Every square has 20 buddies.

The coordinates of cells in a Sudoku grid.

Copyright © 2007
Written by Ko Chung Wong