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Rules to solve Sudoku puzzles


1. Sudoku Terminology
2. Rules of Sudoku
3. Rules for Sudoku variations
3.1 Overlap Sudoku
3.2 Tanto or Even/Odd Sudoku
3.3 Hyper Sudoku
3.4 Sudoku X

Do you want to learn how to start solving Sudoku puzzles? Did a Sudoku variation leave you puzzled? Fear not. You have come to the right place. Here we will explain the rules of Sudoku, and in no time you will be on your way to solving sudoku puzzles yourself!!

1. Terminology

Before we go further, let's establish some terminologies which we will be using in later sections.

Cell: A cell is a square box in a sudoku game in which a number/letter can be entered. A cell can be either pre-defined or empty. Pre-defined cells are fixed in a sudoku and their values cannot be changed. Empty cells are empty to begin with, and the user/player must fill in the correct value.

Block: A block (or a cell-block) is a group of cells in a Sudoku game. A block can be distinguished from normal cells by its thicker border. A block can be a regular block of NxN cells in the form of a square, or can be of an irregular shape. Both kinds are blocks nonetheless.

Row: A row is a horizontal collection of cells in a Sudoku game.

Column: A column is a vertical collection of cells.

The following figures show the above concepts on a Sudoku game grid:






2. Rules of Sudoku

Now, the easier part:
  • An N celled block (for example, a 3x3 block will have 9 cells) should be filled with numbers from 1 through N. This way, a 3x3 block will be filled with numbers 1 through 9.
  • No two cells within one block can have the same number.
  • No two cells in a row can have the same number.
  • No two cells in a column can have the same number.

To make the above rules clearer, following figures illustrate some wrong entries:


That's it!! You're now officially qualified to solve Sudoku games. For tips on how to go about solving Sudoku, see our Tips & Strategies page.

3. Rules for Sudoku variations

Sudoku variations, many of which seem perplexing at first, are infact quite easy to learn. This section describes the rules to work with Sudoku variations that one can play with on SudokuSplashZone.

3.1 Overlap Sudoku

In an overlap sudoku, multiple regular 9x9 sudoku puzzles overlap each other to form a single game. The individual 9x9 sudoku puzzles should be solved just like a normal 9x9 sudoku. The blocks which belong to or are common to more than one 9x9 grid must satisfy the conditions in all the 9x9 grids that they belong to.

3.2 Tanto or Even/Odd Sudoku

In a tanto sudoku (also known as Even/Odd Sudoku), some cells are marked with a different color (usually gray). All these cells have either even values, or all these cells have odd values. The figure below shows a Tanto wherein the gray cells all have even values. All the remaining cells have odd values.



Apart from this additional constraint, a Tanto Sudoku is like any other Sudoku. This may lead to situations in which it is easier to deduce a Tanto Sudoku cell value. For example, suppose that while solving a Tanto, a cell's possible values are narrowed down to, say 1, 2 and 5. Now in a regular Sudoku, you would have to use pencil marks and retain the cell's values as 1,2,5 until something more becomes known. But in a Tanto, if the cell happens to be an even cell, one could immediately reject 1 and 5 (because they are odd :-) and fix the cell value as 2.

3.3 Hyper Sudoku

A hyper sudoku has additional shaded blocks which usually overlap the underlying regular sudoku blocks. These additional shaded 3x3 blocks must also be satisfied (i.e., they must have all numbers from 1 through 9). The figure below shows a part of a hyper sudoku. In this figure, blocks 1, 2, 3 and 4 are 3x3 blocks that are found in any regular sudoku. In addition to these blocks, this hyper sudoku has a hyper block, which is shaded in gray.


3.4 Sudoku X

A Sudoku-X has two additional blocks which form an X-shape. Each of these blocks spans the diagonal of a 9x9 sudoku grid, as shown in the figure below.


Apart from the regular Sudoku blocks, each of these diagonal shaded blocks must be satisfied too (i.e., each diagonal block must have all numbers from 1 through 9).