Sudoku, typically a 9x9 number puzzle, whose aim is to fill every square of the grid so that every unit ( column, row or 3x3 block ) contains the digits 1 to 9 and each digit must occur once and once only in each unit. Although the rule is simple, the puzzle can be very difficult to solve.

( "sudoku" in Japanese, actually in Chinese Simplified, since a lot of Japanese characters, including this word, came directly from Chinese )

To understand how to play and solve a sudoku puzzle, let's first introduce the terms we use on this web site and in our software iSudoku.

The whole puzzle area is called a grid. Typically it is composed of 9x9=81 cells (or squares). Some of the cells are already assigned with values, while others are left blank for players to solve.

A row refers to all cells in a horizontal line. And a column refers to all cells in a vertical line. There are, obviously, totally 9 rows and 9 columns in the puzzle above, and each contains 9 cells. To save confusion, rows and columns are refered by Capitalised letters and numbers respectively. For example, cell [G6] means the cell in Row G and Column 6, where there is a value 7 located.

The term block is used for a set of nine adjacent cells. Alternate colors are used to mark the blocks in a puzzle grid, as shown in the figure above. For example, Block at cell [A1] refers to the top-left block, which starts at Row A and Column 1.

Any one column, row or block is called a unit (or a group). Each unit of nine cells must have a unique occurence of numbers 1 to 9.

People may think the difficulty of a puzzle is based on the quantity of initial given numbers. This is, however, not always the case. Sometimes, a puzzle with more given numbers may probably be more difficult than another one with less given numbers.

Practically, one of the measures to establish the level of difficulty is to find out which and how many sudoku strategies are involved in solving a sudoku puzzle. The easiest puzzle may require only the very basic techniques to solve. A harder puzzle, in the other hand, tends to require very advanced strategies.

People are always not satisfied with what they already have. Even though the 9x9 grid with 3x3 blocks is by far the most common, more and more sudoku variations appear and become part of the sudoku family.

A common variant is to change the size of the sudoku grid. Sample puzzles can be 4x4 grids, 6x6 grids, 12x12 grids, 16x16 grids, 25x25 grids and even 100x100 grids.

Another variant is to enforce additional restrictions on the placement of numbers beyond the usual row, column and block requirements. For example, Sudoku X requires the numbers in the main diagonals of the grid also be unique. Killer Sudoku, however, requires the values in a cage (a group of cells surrounded by dashed lines) must be unique and sum up to the total specified in the upper right of the cage.